Photoelectric conversion element

ABSTRACT

A photoelectric conversion element having a composite dye and an n-type semiconductor, the composite dye having a plurality of component dyes which have different excitation levels and which are chemically bonded to each other to form a straight chain or branched structure for transferring an electron therethrough, wherein the straight chain or branched structure is, at one end thereof, secured to the n-type semiconductor and has, at least at one other end thereof, a free end, wherein, in the straight chain or branched structure, the plurality of component dyes are arranged in an order such that the excitation levels of the plurality of component dyes are decreased as viewed from the one end of the structure toward the at least one other end of the structure.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a photoelectric conversion element.More particularly, the present invention is concerned with aphotoelectric conversion element comprising a composite dye and ann-type semiconductor, the composite dye comprising a plurality ofcomponent dyes which have different excitation levels and which arechemically bonded to each other to form a straight chain or branchedstructure for transferring an electron therethrough, wherein thestraight chain or branched structure is, at one end thereof, secured tothe n-type semiconductor and has, at least at one other end thereof, afree end, and wherein the plurality of component dyes are arranged in anorder such that the excitation levels of the plurality of component dyesare decreased as viewed from the one end of the structure toward the atleast one other end of the structure. The photoelectric conversionelement of the present invention exhibits excellent photoelectricconversion properties, especially high efficiency in converting solarenergy to electric energy (i.e., high energy conversion efficiency), anda dye-sensitized solar battery can be easily produced therefrom.Therefore, the photoelectric conversion element of the present inventioncan be advantageously used for a dye-sensitized solar battery and thelike.

The present invention is also concerned with a dye-sensitized solarbattery using the photoelectric conversion element.

2. Prior Art

Consumption of energy is indispensable to civilized society. Most of theenergy which is consumed by civilized society is derived from fossilfuels, in which sunray energy has been accumulated over many years. Inrecent years, the problem that the amount of fossil fuels available isbeing reduced and the problem that the burning of fossil fuels causesglobal warming have arisen, and there is an increasing fear that theseproblems will be obstacles to the sustainable development of humansociety.

For solving the above-mentioned problems, various studies have been madeto directly utilize sunray energy. Among these studies, the studies onsolar batteries have been vigorously made, because solar batteriesexhibit high efficiency in converting solar energy to electric energy(i.e., high energy conversion efficiency). Among the solar batteries,special attention has been paid to a dye-sensitized solar battery, whichuses a photosensitizer, such as a dye, and which is capable ofefficiently taking out electrons from the photosensitizer by theirradiation of the photosensitizer with sunray. Specifically, sinceMichael Gratzel et al. reported a system which uses a dye-sensitizedsolar battery having an energy conversion efficiency of more than 7%(see Nature 1991, 353, 737), a dye-sensitized solar battery has drawnspecial attention as the next-generation solar battery which can beproduced at a low cost without use of a complicated method.

In general, a dye-sensitized solar battery has the following structure.A substrate (electroconductive support) is laminated on a support madeof glass, a polymer or the like, wherein the substrate has coatedthereon an indium oxide membrane (e.g., an ITO (indium tin oxide)membrane) or fluorine-doped tin oxide (FTO) membrane having excellentconductivity and excellent transparency. On the substrate is laminated aporous titanium oxide membrane, as an n-type semiconductor, which is acheap material and has a size of several tens of nanometers, therebyforming a laminate as a photo-anode. A thin layer of platinum islaminated on a substrate which is substantially the same as theabove-mentioned substrate, thereby forming a laminate as a cathode.Between the photo-anode and the cathode is interposed an electrolyticsolution containing a redox couple, such as iodine, thereby forming astructure in which a photo-anode and a cathode face each other throughan electrolytic solution. In this structure, the photo-anode has carriedthereon a dye (such as a complex dye) as a photosensitizer for thepurpose of absorbing visible light from sunray to thereby generateexcited electrons from the photosensitizer, so that the photo-anodefunctions as a photoelectric conversion element.

The excited electrons generated from the photosensitizer are transferredto the n-type semiconductor, and further transferred to the cathodethrough a conductor which connects the photo-anode and cathode. Theexcited electrons having been transferred to the cathode reduce theelectrolytic solution and, in turn, the electrolytic solution reducesthe photosensitizer having been oxidized by the emission of electronsfrom the photosensitizer. By repeating the above-mentioned series ofoperations, the dye-sensitized solar battery works.

A photosensitizer, such as a dye, can absorb light having wavelengthswithin a certain range. When the photosensitizer is irradiated with suchlight having wavelengths within the range, the photosensitizer receivesthe energy of photons. As a result, electrons in the ground state,contained in the photosensitizer, are excited and transferred to excitedstates. In general, excited electrons emit energy in the form of heat orlight, such as fluorescence or phosphorescence, and return to the groundstate. However, when electrons in the photosensitizer are excited,conversion of light energy to electric energy (i.e., photoelectricconversion) is done by taking out the excited electrons from thephotosensitizer.

As seen from the above, a photosensitizer plays an important part in theconversion of light energy to electric energy. Therefore, studies onphotosensitizers have been vigorously made.

When the function of a photosensitizer is discussed in terms ofmolecules, the photosensitizer generally receives one photon to exciteone electron contained therein. The longer the wavelength of a light,the smaller the energy of the light. It follows from this that aphotosensitizer which absorbs a long-wavelength light to exciteelectrons contained therein (i.e., make electrons contained thereintransferred to excited states) can excite electrons contained therein byabsorbing a low energy light. Therefore, the photosensitizer can absorba wide range of light, which ranges from a long-wavelength light to ashort-wavelength light, which has a large energy. In the application ofa solar battery, for taking out a number of electrons (i.e., obtaining ahigh electric current), it is important to effectively utilize a widerange of light contained in sunray, which has a wide distribution oflight wavelengths. In view of this, various attempts have been made todevelop a photosensitizer capable of absorbing longer wavelength lightfrom sunray.

For this purpose, i.e., for obtaining a photosensitizer capable ofabsorbing longer wavelength light from sunray, it is generally attemptedto enlarge a conjugate structure. For example, Japanese PatentApplication Prior-to-Examination Publication (Tokuhyo) No. 2002-512729(corresponding to WO98/50393 and U.S. Pat. No. 6,245,988) discloses atechnique in which a single nuclear complex dye having a tridentateligand is used. On the other hand, Inorg. Chem. 2002, 41, 367 disclosesa technique in which a single nuclear complex dye having a quadridentateligand is used. Further, J. Phys. Chem. B 2003, 107, 597 discloses atechnique in which an organic dye having a conjugate structure is used.

Moreover, for obtaining a photosensitizer capable of absorbing longerwavelength light from sunray, a technique using a multinuclear complexhaving a plurality of metals is disclosed in Unexamined Japanese PatentApplication Laid-Open Specification No. 2000-323191 (corresponding toEP1052661). Also, for using a plurality of dyes in combination, atechnique in which a plurality of dye layers are laminated is disclosedin Unexamined Japanese Patent Application Laid-Open Specification No.2000-195569, and a technique in which a plurality of dyes are associatedwith each other is disclosed in Unexamined Japanese Patent ApplicationLaid-Open Specification No. 2002-343455.

However, the above-mentioned techniques, i.e., the techniques in which asingle dye is used for absorbing longer wavelength light from sunray,and the techniques in which a plurality of dyes are used in combination(wherein the dyes receive excited electrons having the same energy levelfrom the electrolyte and transfer the excited electrons to the n-typesemiconductor), have a theoretical limit on the energy conversionefficiency when electrons are taken out from sunray having a widedistribution of wavelengths. The reason for this is as follows. As a dyeabsorbs light having longer wavelengths from sunray, the number ofelectrons taken out from the dye is increased, so that a larger electriccurrent can be obtained. However, light having a long wavelength has asmall energy, so that the energy which can be used for transferring anelectron to an excited state is inevitably small. Therefore, a highvoltage cannot be obtained.

As mentioned above, in the case of a general photosensitizer, the energyof only one photon is used for exciting a single electron (one-photonabsorption). However, in the case of a photosensitizer comprising aspecific compound, the energies of two photons can be used for excitinga single electron (two-photon absorption) (see Science 1998, 281, 1653).In such case, it becomes possible to transfer an electron to a highenergy level by using only low-energy light having long wavelengths and,hence, the above-mentioned theoretical limit can be overcome. Thetechnique of the two-photon absorption is a technique in which anelectron having been excited in a molecule is further excited in themolecule. An electron in an excited state returns to the ground state ina short period of time. Therefore, in the technique of the two-photonabsorption, generally, the electron is transferred to a quasi-stableexcited state (such as a triplet state) so that the life time of theexcited electron (i.e., excitation lifetime) becomes longer, therebyassuring a time sufficient for causing a second excitation of theelectron. However, even in the technique of the two-photon absorption,it is necessary for a single molecule to absorb light twice in a shortperiod of time and, hence, the probability that the second excitation ofthe electron occurs is small, so that it is difficult to take out anumber of electrons from the photosensitizer. Therefore, it is difficultto apply the two-photon absorption technique to fields (such as thefield of solar batteries) in which it is necessary to take out a numberof electrons. Thus, the fields to which it is attempted to apply thetwo-photon absorption technique are limited to the field ofpolymerization initiators (see Nature 1999, 398, 51) and the field ofphotosensors (see Unexamined Japanese Patent Application Laid-OpenSpecification No. 2001-210857).

In addition, as a technique for efficiently taking out solar energy, J.He et al. propose a technique in which two electrodes facing each otherare respectively provided with n-type and p-type semiconductor layers,wherein the semiconductor layers are respectively sensitized by dyeshaving different excitation levels (see Solar Energy Materials & SolarCells 2000, 62, 265). However, this technique has a problem in that eachof the provision of a semiconductor layer and the adsorption of a dyemust be performed a plurality of times, thereby rendering combersome theproduction of a system used in the technique.

SUMMARY OF THE INVENTION

In this situation, the present inventors have made extensive andintensive studies with a view toward developing a photoelectricconversion element which exhibits excellent photoelectric conversionproperties, especially high efficiency in converting solar energy toelectric energy (i.e., high energy conversion efficiency) and from whicha dye-sensitized solar battery can be easily produced. As a result, ithas unexpectedly been found that such a photoelectric conversion elementis realized by a photoelectric conversion element comprising a compositedye and an n-type semiconductor, the composite dye comprising aplurality of component dyes which have different excitation levels andwhich are chemically bonded to each other to form a straight chain orbranched structure for transferring an electron therethrough, whereinthe straight chain or branched structure is, at one end thereof, securedto the n-type semiconductor and has, at least at one other end thereof,a free end, and wherein the plurality of component dyes are arranged inan order such that the excitation levels of the plurality of componentdyes are decreased as viewed from the one end (secured to the n-typesemiconductor) of the structure toward the at least one other end of thestructure. Based on this finding, the present invention has beencompleted.

Accordingly, it is an object of the present invention to provide aphotoelectric conversion element which exhibits high efficiency inconverting solar energy to electric energy (i.e., high energy conversionefficiency) and from which a dye-sensitized solar battery can be easilyproduced.

It is another object of the present invention to provide adye-sensitized solar battery using the photoelectric conversion element.In this solar battery, as the electrolyte, an electrolyte having anappropriate redox potential is used for adjusting the potential of thecounter electrode, so that a very high voltage can be obtained.

The foregoing and other objects, features and advantages of the presentinvention will be apparent from the following detailed description takenin connection with the accompanying drawings and appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 shows an example of the results of the cyclic voltammetryperformed with respect to a composite dye used in the present invention;

FIG. 2 shows a diagrammatic view explaining the electron transferachieved by the use of the photoelectric conversion element of thepresent invention and the resulting shift of the electron-deficientorbital from a photoabsorption portion having a higher energy level to aphotoabsorption portion having a lower energy level;

FIG. 3 shows a diagrammatic view explaining the compatibility ofabsorption of long-wavelength lights with high voltage, which isachieved by the use of the photoelectric conversion element ordye-sensitized solar battery of the present invention;

FIG. 4 shows a diagrammatic view explaining an example of the structureof a multinuclear complex used in the present invention;

FIG. 5 shows a diagrammatic view explaining the concepts of the energylevels and electron orbitals with respect to the components of themultinuclear complex shown in FIG. 4;

FIG. 6 shows a diagrammatic view explaining an example of the structureof a heterocycle having a conjugated double bond, wherein theheterocycle is contained in a multinuclear complex used in the presentinvention;

FIGS. 7( a), 7(b) and 7(c) show representative examples of bridgingligands contained in multinuclear complexes used in the presentinvention;

FIGS. 8( a) and 8(b) show other examples of bridging ligands containedin multinuclear complexes used in the present invention;

FIG. 9 shows an example of the structure of a dye-sensitized solarbattery of the present invention;

FIG. 10 shows the results of the measurement by matrix-assisteddesorption ionization/time-of-flight mass spectrometry (MALDI-TOF-MS)performed with respect to the complex dye produced in Example 1;

FIG. 11 shows the representative structure of the composite dye producedin Example 1;

FIG. 12 shows the results of the measurement by matrix-assisteddesorption ionization/time-of-flight mass spectrometry (MALDI-TOF-MS)performed with respect to the complex dye precursor produced in Example2;

FIG. 13 shows the representative structure of the composite dyeprecursor produced in Example 2;

FIG. 14 shows the results of the measurement by matrix-assisteddesorption ionization/time-of-flight mass spectrometry (MALDI-TOF-MS)performed with respect to the complex dye produced in Example 2;

FIG. 15 shows the representative structure of the composite dye producedin Example 2;

FIG. 16 shows the results of the measurement, performed in Example 7, ofthe change (with the change of the intensity of light ray) in the ratioof the electric current generated in a dye-sensitized solar batterycontaining a composite dye (wherein the electric current is generated byirradiating the solar battery with the light ray) to the electriccurrent generated in a dye-sensitized solar battery containing a singledye (wherein the electric current is generated by irradiating the solarbattery with the light ray);

FIG. 17 shows the results of the measurement, performed in Example 8, ofthe change (with the change of the intensity of light ray) in the ratioof the electric current generated in a dye-sensitized solar batterycontaining a composite dye (wherein the electric current is generated byirradiating the solar battery with the light ray) to the electriccurrent generated in a dye-sensitized solar battery containing a singledye (wherein the electric current is generated by irradiating the solarbattery with the light ray); and

FIG. 18 shows the results of the measurement, performed in ReferenceExample 1, of the change (with the change of the intensity of light ray)in the ratio of the electric current generated in a dye-sensitized solarbattery containing a single dye (wherein the electric current isgenerated by irradiating the solar battery with the light ray) to theelectric current generated in a dye-sensitized solar battery containinganother single dye (wherein the electric current is generated byirradiating the solar battery with the light ray).

DETAILED DESCRIPTION OF THE INVENTION

According to the present invention, there is provided a photoelectricconversion element comprising a composite dye and an n-typesemiconductor,

the composite dye comprising a plurality of component dyes which havedifferent excitation levels and which are chemically bonded to eachother to form a straight chain or branched structure for transferring anelectron therethrough, wherein the straight chain or branched structureis, at one end thereof, secured to the n-type semiconductor and has, atleast at one other end thereof, a free end,

wherein, in the straight chain or branched structure, the plurality ofcomponent dyes are arranged in an order such that the excitation levelsof the plurality of component dyes are decreased as viewed from the oneend (secured to the n-type semiconductor) of the structure toward the atleast one other end of the structure.

For easier understanding of the present invention, the essentialfeatures and various preferred embodiments of the present invention areenumerated below.

1. A photoelectric conversion element comprising a composite dye and ann-type semiconductor,

the composite dye comprising a plurality of component dyes which havedifferent excitation levels and which are chemically bonded to eachother to form a straight chain or branched structure for transferring anelectron therethrough, wherein the straight chain or branched structureis, at one end thereof, secured to the n-type semiconductor and has, atleast at one other end thereof, a free end,

wherein, in the straight chain or branched structure, the plurality ofcomponent dyes are arranged in an order such that the excitation levelsof the plurality of component dyes are decreased as viewed from the oneend of the structure toward the at least one other end of the structure.

2. The photoelectric conversion element according to item 1 above,wherein each component dye of the composite dye comprises a metal atomhaving a ligand coordinated thereto, so that the composite dye iscomprised of a multinuclear complex comprising a plurality of metalatoms and a plurality of ligands including at least one bridging ligand,wherein the or each bridging ligand is positioned between mutuallyadjacent metal atoms in the multinuclear complex to thereby bridge themutually adjacent metal atoms.3. The photoelectric conversion element according to item 2 above,wherein the or each bridging ligand in the multinuclear complex has anasymmetric structure.4. The photoelectric conversion element according to item 3 above,wherein the or each bridging ligand in the multinuclear complexcomprises a heterocyclic segment having a conjugated double bond and,bonded to the heterocyclic segment, a non-heterocyclic segment, tothereby form the asymmetric structure,

wherein the heterocyclic segment is positioned in the or each bridgingligand on a side thereof remote from the n-type semiconductor ascompared to the non-heterocyclic segment, wherein a heteroatom in theheterocyclic segment is positioned on a side thereof remote from then-type semiconductor.

5. A dye-sensitized solar battery comprising:

an electrode comprised of the photoelectric conversion element of anyone of items 1 to 4 above,

a counter electrode, and

an electrolyte interposed between the photoelectric conversion elementand the counter electrode,

wherein the dye-sensitized solar battery becomes operable when theelectrode comprised of the photoelectric conversion element and thecounter electrode are connected to each other through anelectroconductive material which is positioned outside of theelectrolyte.

6. The dye-sensitized solar battery according to item 5 above, whereinthe counter electrode exhibits a potential of −0.2 V or more relative tothe redox potential of silver/silver ion.

Hereinbelow, the present invention is described in detail.

The photoelectric conversion element of the present invention comprisesa composite dye and an n-type semiconductor.

The composite dye comprises a plurality of component dyes which havedifferent excitation levels and which are chemically bonded to eachother to form a straight chain or branched structure for transferring anelectron therethrough, wherein the straight chain or branched structureis, at one end thereof, secured to the n-type semiconductor and has, atleast at one other end thereof, a free end. In the straight chain orbranched structure, the plurality of component dyes are arranged in anorder such that the excitation levels of the plurality of component dyesare decreased as viewed from the one end of the structure toward thefree end (i.e., the at least one other end) of the structure.

In the present invention, the term “excitation level” means the orbitallevel (of a ground state) at which the dye (component dye or compositedye) absorbs light having a wavelength equal to or higher than that ofvisible light to thereby get strongly excited. In general, the orbitallevel of such ground state is the highest occupied molecular orbital(HOMO). So long as the transition of an electron from the highestoccupied molecular orbital can be observed, the highest occupiedmolecular orbital is regarded as the excitation level. In the presentinvention, in which a plurality of component dyes are chemically bondedto each other to form a composite dye, the excitation level of acomponent dye means the orbital level (of a ground state) at which thecomponent dye absorbs light having a wavelength within the range equalto or higher than that of visible light to thereby get strongly excitedwhen the component dye is present alone without forming a composite dyeby the chemical bonding thereof to another component dye.

In the present invention, the expression “the component dyes havedifferent excitation levels” means that the difference between theexcitation levels of any two component dyes is 0.05 eV or more. It ispreferred that the difference between the excitation levels of any twocomponent dyes is 0.1 eV or more, more advantageously 0.2 eV or more,most advantageously 0.4 eV or more. With respect to the upper limit ofthe difference between the excitation levels of two component dyes,there is no particular limitation. However, when the difference betweenthe excitation levels of two component dyes is large, it is impossibleto introduce an excited electron to the n-type semiconductor by the useof visible light. Therefore, it is preferred that the difference betweenthe excitation levels of any two component dyes is 3 eV or less, moreadvantageously 2.5 eV or less, most advantageously 2 eV or less.

In the present invention, the expression “the excitation level is low”means that the energy level is in a stable state, and that the value ofthe electric potential, as measured by the below-mentionedelectrochemical method, is large.

In the present invention, the excitation levels of the component dyesare measured and the decreasing or increasing order thereof is found byelectrochemical methods, such as cyclic voltammetry. With respect tocyclic voltammetry, reference is made to Allen J. Bard et al.,“Electrochemical Methods: Fundamentals and Applications”, John Wiley &Sons, 1980. Specifically, cyclic voltammetry is performed as follows.The oxidation potential of each component dye (photoabsorption portion)(or a derivative or precursor thereof) is individually measured bycyclic voltammetry. From the found value of the oxidation potential ofthe component dye (photoabsorption portion), the excitation level of thecomponent dye (photoabsorption portion) is found, wherein the results ofthe cyclic voltammetry performed with respect to the composite dye aretaken into consideration. In the measurements by cyclic voltammetry, thesame electrode, the same solvent and the same voltage scanning rate areused. However, when it is impossible to use the same solvent in view ofthe solubility of the component dyes, the decreasing or increasing orderof the excitation level of the component dyes can be determined asfollows. A specific component dye (or a derivative or precursor thereof)is chosen, and the oxidation potential thereof is measured using varioussolvents. The oxidation potential of the component dye varies dependingon the type of the solvent used. From the difference in the oxidationpotential due to the type of the solvent, the oxidation potentials ofthe remainder of the component dyes can be calculated. In some cases, itis possible that the excitation level of a component dye in thecomposite dye is shifted as compared to the excitation level of thecomponent dye prior to forming a chemical bond with another componentdye. In such a case, the shift of the excitation level of the componentdye can be found, based on the changes in the ultraviolet/visiblespectra of component dyes (or derivatives or precursors thereof) and ofthe composite dye.

When it is difficult to determine the decreasing or increasing order ofthe excitation levels of the component dyes by cyclic voltammetry, thedecreasing or increasing order of the excitation levels of the componentdyes can be determined by calculating the highest occupied molecularorbital (HOMO) by a computational chemistry method based on a theory,such as the density functional theory (DFT). When this method is used,it is necessary to choose a technique in which, with respect toconventional compounds, the ionization potentials thereof and thedecreasing or increasing order of the highest occupied molecularorbitals thereof can be correctly calculated. For example, when each ofthe component dyes (or derivatives or precursors thereof) or thecomposite dye is a complex, the calculation can be performed byeffectively using a hybrid type functional, such as B3LYP or PBE1PBE.

When, for the determination of the decreasing or increasing order of theexcitation levels of the component dyes, it is necessary to take intoconsideration the change of the wavelengths of light absorbed by thecomponent dyes, it is preferred to use a computational chemistry methodbased on the time dependent density functional theory (TDDFT). Also inthis case, it is necessary to choose a technique in which, with respectto conventional compounds, the wavelengths of light absorbed thereby orthe decreasing or increasing order thereof can be correctly calculated.

In some cases, if desired, there can be chosen a method in which thedecreasing or increasing order of the excitation levels of the componentdyes is determined using an ionization potential measuring apparatus.

FIG. 1 shows an example of the results of cyclic voltammetry.Specifically, FIG. 1 shows the results of cyclic voltammetry performedwith respect to the composite dye produced in Example 2. As seen fromFIG. 1, oxidation waves are observed in different levels. This meansthat the component dyes in the composite dye produced in Example 2 havedifferent excitation levels.

In the present invention, the term “component dye” means a compoundwhich is dyed by the absorption of visible light. Examples of componentdyes include organic dyes and complex dyes. Examples of organic dyesinclude compounds which can be obtained as commercially available dyes,such as a cyanine dye, a cumarin dye, a spiropyran dye, an azo dye and axanthene dye, and derivatives of the compounds. The term “complex dye”means a compound comprised of at least one metal atom and at least oneligand, which is dyed by the absorption of visible light. In the presentinvention, a plurality of component dyes are chemically bonded to form acomposite dye.

In the present invention, the expression “chemically bonded” means“bonded by a chemical bond, such as a covalent bond, an ionic bond or acoordinate bond”. The presence of a chemical bond can be confirmed by,for example, a method in which a chemical bond is confirmed by any ofthe below-mentioned measurements, or a method in which the solvent for acomponent dye (a photoabsorption portion or component dye precursor)which is not chemically bonded to another component dye is used forwashing the composite dye to confirm that the composite dye is notdissolved in the solvent.

As mentioned above, the composite dye forms a straight chain or branchedstructure for transferring an electron therethrough. The structure is,at one end thereof, secured to the n-type semiconductor and has, atleast at one other end thereof, a free end.

With respect to the above-mentioned structure, the expression “securedto the n-type semiconductor” means that the structure is not detachedfrom the n-type semiconductor and that the transfer of an electron fromthe structure to the n-type semiconductor is possible. Examples of modesin which the structure is secured to the n-type semiconductor include amode in which the structure is, at one end thereof, physically adsorbedon the n-type semiconductor, a mode in which the structure is, at oneend thereof, chemically adsorbed on the n-type semiconductor, and a modein which the structure is, at one end thereof, chemically bonded to then-type semiconductor. Examples of such chemical bonds include an esterlinkage, a phosphoric ester linkage, a coordinate bond and an ionicbond.

In the photoelectric conversion element of the present invention, thecomponent dyes are arranged in an order such that the excitation levelsof the component dyes are decreased as viewed from the end secured tothe n-type semiconductor toward the free end. By virtue of thearrangement of the component dyes in the above-mentioned order, highenergy conversion efficiency can be achieved. The reason for this is asfollows.

When the composite dye is irradiated with light rays, electrons in thecomposite dye are excited. The excited electron in the component dyewhich is secured to the n-type semiconductor (hereinafter, thiscomponent dye is frequently referred to as “first photoabsorptionportion”) can be easily transferred to the n-type semiconductor. As aresult, the first photoabsorption portion turns into anelectron-deficient state. When a component dye which is adjacentlybonded to a component dye in an electron-deficient state gets excited,an electron in the excited component dye can be easily transferred tothe electron-deficient orbital in the component dye in anelectron-deficient state. Thus, the electron-deficient state shifts fromthe component dye secured to the n-type semiconductor (i.e., firstphotoabsorption portion) to a component dye which is chemically bondedto the component dye secured to the n-type semiconductor (hereinafter,the component dye which is adjacently bonded to the firstphotoabsorption portion is frequently referred to as “secondphotoabsorption portion”, and the component dye which is adjacentlybonded to the second photoabsorption portion is referred to as “thirdphotoabsorption portion”, and the component dye bonded to the thirdabsorption portion is referred to as “fourth photoabsorption portion”,and so on). The first photoabsorption portion has the highest excitationlevel, and the excitation level becomes lower in the order of the firstphotoabsorption portion, the second photoabsorption portion, the thirdphotoabsorption portion, and so on (that is, the more distant from then-type semiconductor the photoabsorption portion, the lower theexcitation level thereof). Therefore, it becomes possible tosuccessively shift the electron-deficient orbital from a photoabsorptionportion having a higher energy level to a photoabsorption portion havinga lower energy level (this is diagrammatically indicated in FIG. 2). Ingeneral, when the excitation level of a component dye is lowered withoutchanging the energy level of the unoccupied orbital of the componentdye, the electron transition width becomes broad, so that the componentdye absorbs only light having a short wavelength. However, when theabove-mentioned composite dye is used, by virtue of the above-mentionedmechanism, it becomes possible to absorb light having a long wavelengthas well as light having a short wavelength, so that a broad electrontransition range can be achieved. Therefore, by using a photoelectricconversion element comprised of the above-mentioned composite dye and ann-type semiconductor, there can be obtained a high performancedye-sensitized solar battery having the capability of absorbing lighthaving a long wavelength and exhibiting a high voltage, whereinelectricity can be generated by using an electrolyte having a low energylevel, while maintaining the properties of absorbing visible light and,if desired, long-wavelength light, such as near-infrared light (this isdiagrammatically indicated in FIG. 3). The composite dye used in thepresent invention comprises a plurality of component dyes which arechemically bonded to each other, so that the above-mentioned transfer ofexcited electrons occurs with a high probability. It is especiallypreferred that the composite dye has an asymmetric structure, so thatexcited electrons are easily transferred from the component dye at afree end of the composite dye to the component dye at the secured end ofthe composite dye. In such a case, the above-mentioned transfer ofexcited electrons occurs with a higher probability.

FIG. 5 is a diagrammatic view explaining the concepts of the energylevels and electron orbitals with respect to each of the component dyesof the multinuclear complex shown in FIG. 4. When the multinuclearcomplex is irradiated with light rays, an electron transition occursfrom the occupied electron orbital ascribed to metal atom M₀ to theunoccupied electron orbital ascribed to ligand L₀ (in FIG. 5, thiselectron transition is indicated by symbol “(A)”), and the excitedelectron is transferred from ligand L₀ to the n-type semiconductor (inFIG. 5, this electron transfer is indicated by symbol “(B)”). By theelectron transition from the occupied electron orbital ascribed to metalatom M₀ to the unoccupied electron orbital ascribed to ligand L₀, theoccupied electron orbital ascribed to metal M₀ turns into anelectron-deficient state. Then, when an electron transition occurs fromthe occupied electron orbital ascribed to metal atom M to the unoccupiedelectron orbital ascribed to bridging ligand BL (in FIG. 5, thiselectron transition is indicated by symbol “(C)”), the excited electronis transferred from the electron orbital ascribed to bridging ligand BLto the electron-deficient orbital ascribed to metal atom M₀, wherein theelectron orbital ascribed to bridging ligand BL is located adjacently tothe electron-deficient orbital ascribed to metal atom M₀ and has anenergy level approximate to the electron-deficient orbital ascribed tometal atom M₀ (in FIG. 5, this electron transfer is indicated by symbol“(D)”). As a result, the electron-deficient state shifts from theelectron orbital ascribed to metal atom M₀, which has a higher energylevel, to the electron orbital ascribed to metal atom M, which has alower energy level. Therefore, when the multinuclear complex as acomposite dye is used in a dye-sensitized solar battery, such anelectron-deficient orbital ascribed to metal atom M can receive anelectron from an electrolyte which has an energy level lower than thatof the electron-deficient orbital ascribed to metal atom M.

By successively performing the above-mentioned operation in which anexcited electron is transferred to an orbital having an electron emittedtherefrom (i.e., electron-deficient orbital) to thereby successivelyshift the electron-deficient orbital to a lower energy level, it becomespossible to functionalize a stepwise multiphoton absorption process inwhich an electron transition from a ground state of each component dyein the composite dye is used. This multiphoton absorption process usedin the present invention is advantageous as compared to a conventionalmultiphoton absorption process in which an excited electron is furtherexcited. The reason for this is that, in the multiphoton absorptionprocess used in the present invention, the stability of an excitedelectron (i.e., lifetime of an excited electron) does not influence theprobability of the occurrence of a subsequent electron transition, sothat it becomes easier to functionalize the multiphoton absorptionprocess.

With respect to the number of the component dyes (photoabsorptionportions) contained in the composite dye, there is no particularlimitation. The larger the number of the component dyes, the higher thetheoretical upper limit of the conversion efficiency of the multiphotonabsorption process. However, when the number of the component dyes(photoabsorption portions) is 4 or more, the improvement in thetheoretical upper limit of the conversion efficiency of the multiphotonabsorption process, achieved by the increase in the number of thecomponent dyes, becomes small. Therefore, from the viewpoint of thebalance of the difficulty in the production of the composite dye and thetheoretical conversion efficiency, it is preferred that the number ofthe component dyes (photoabsorption portions) which are to be chemicallybonded is 3 or less, more advantageously 2.

Since the composite dye plays a part in the introduction of an electroninto the n-type semiconductor, it is preferred that the firstphotoabsorption portion has a functional group which bonds to the n-typesemiconductor or particles thereof. Specifically, it is preferred thatthe first photoabsorption portion has a functional group selected fromthe group consisting of a carboxylic acid group, a carboxylic acid saltgroup, a sulfonic acid group, a sulfonic acid salt group, a phosphoricacid group and a phosphoric acid salt group. Further, with respect to afunctional group, such as a carboxylic ester group, a phosphoric estergroup or a sulfonic ester group, the functional group can also be usedif, when employed, the functional group has the capability of forming alinkage which is substantially the same as one of the above-mentionedlinkages.

Examples of composite dyes include a composite dye comprised of amultinuclear complex, and a composite dye comprised of organic dyeswhich are bonded to each other, wherein each of the organic dyes has a πconjugate bond and, if desired, has bonded thereto a functional group.Among these composite dyes, a multinuclear complex is preferred.

With respect to a composite dye comprised of a multinuclear complex,explanation is given below. When a composite dye is comprised of amultinuclear complex, each component dye of the composite dye comprisesa metal atom having a ligand coordinated thereto, so that the compositedye is comprised of a multinuclear complex comprising a plurality ofmetal atoms and a plurality of ligands including at least one bridgingligand, wherein the or each bridging ligand is positioned betweenmutually adjacent metal atoms in the multinuclear complex to therebybridge the mutually adjacent metal atoms.

The reason why a composite dye comprised of a multinuclear complex ispreferred is as follows. When the composite dye comprises a multinuclearcomplex, with respect to each component dye of the composite dye, thedifference in the location of the electrons in the component dye betweenthe ground state and an excited state thereof is distinguishable (forexample, there is a case where, in the ground state, the electrons arepresent mainly in the metal atom, and, in the excited state, theelectrons are present mainly in the ligand), differing from the case ofan organic dye. Therefore, it becomes easy to clearly separate thefunction of bonding the component dyes from the function of introducingexcited electrons to the component dyes. Further, when the composite dyeis comprised of a multinuclear complex, the composite dye is in a stablestate after it has released an electron, as compared to the case of anorganic dye. Therefore, the composite dye can be preferably used in thepresent invention wherein it is intended to successively perform thetransfer of electrons. When the complex dye is used in a photoelectricconversion element with an n-type semiconductor, the complex dye in thephotoelectric conversion element is in a stable state after it hasreleased an electron. Therefore, the use of the complex dye isespecially preferred in the present invention in which the stepwisemultiphoton absorption process is employed.

As mentioned below, in many cases, a multinuclear complex as a complexdye is produced by a ligand exchange reaction between a plurality ofcomplex dyes as component dyes. During this ligand exchange reaction, apart of the complex dyes (component dyes) (i.e., ligands contained inthe complex dyes) are detached therefrom. The multinuclear complexproduced by such exchange reaction is regarded as a composite dye inwhich a plurality of component dyes are chemically bonded to each other,when, with respect to each component dye as a precursor, visible lightabsorption ascribed to an electron transition in a metal atom containedtherein is observed in the multinuclear complex.

It is preferred that the or each bridging ligand (which corresponds tothe ligand represented by “BL” in formula (1) below) in the multinuclearcomplex has an asymmetric structure. By virtue of the asymmetricstructure, it becomes possible to orient the electron transfer in themultinuclear complex, thereby making it possible to efficiently take outan electron generated by an electron transition in a component dye (suchas the second photoabsorption portion) positioned remote from the n-typesemiconductor.

It is preferred that the asymmetric structure of the or each bridgingligand in the multinuclear complex comprises a heterocyclic segmenthaving a conjugated double bond and, bonded to the heterocyclic segment,a non-heterocyclic segment, to thereby form the asymmetric structure,wherein the heterocyclic segment is positioned in the or each bridgingligand on a side thereof remote from the n-type semiconductor ascompared to the non-heterocyclic segment, wherein a heteroatom (an atomother than a carbon atom) in the heterocyclic segment is positioned on aside thereof remote from the n-type semiconductor and is coordinated toa metal atom (hereinafter, a heterocyclic segment having a conjugateddouble bond is frequently referred to simply as “heterocyclic segment”,and a heterocycle having a conjugated double bond is frequently referredto simply as “heterocycle”). The heterocyclic segment may be comprisedonly of a heterocycle or may be comprised of a heterocycle and, bondedthereto, a functional group. When the or each bridging ligand in themultinuclear complex has the above-mentioned asymmetric structure, itbecomes possible to more efficiently take out an electron generated byan electron transition in a component dye positioned remote from then-type semiconductor, since a heterocyclic segment has a highprobability with respect to the “metal to ligand charge transfer”(MLCT).

Therefore, with respect to the photoelectric conversion element of thepresent invention, in which a plurality of component dyes havingdifferent excitation levels are chemically bonded to each other to forma composite dye, for the purpose of improving energy conversionefficiency, it is important that an excited electron obtained by lightabsorption of the component dyes be efficiently taken out as electricenergy through the n-type semiconductor. For realizing this purpose, itis important to efficiently take out an electron generated by electrontransition occurring in the component dye positioned remote from then-type semiconductor. When the composite dye is comprised of amultinuclear complex dye, the probability of the electron transitionfrom the metal on the far side of the n-type semiconductor to thebridging ligand is higher than the probability of the electrontransition from the metal on the near side of the n-type semiconductorto the bridging ligand. Therefore, it is preferred to use such acomposite dye comprised of a multinuclear dye.

As mentioned above, when the heterocyclic segment is coordinated to ametal atom through a heteroatom of the heterocyclic segment, theprobability of an electron transition from the metal atom to theheterocyclic segment is high. Therefore, it is preferred that theheterocyclic segment is positioned in the or each bridging ligand on aside thereof remote from the n-type semiconductor as compared to thenon-heterocyclic segment, wherein a heteroatom (an atom other than acarbon atom) in the heterocyclic segment is positioned on a side thereofremote from the n-type semiconductor.

It is preferred that the above-mentioned multinuclear complex has astructure represented by the following formula (1):(L₀)_(l1)(X₀)_(l2)M₀[(BL)_(m){M(L)_(n1)(X)_(n2)}_(pm)]_(q)  (1)wherein each of L₀ and L independently represents a ligand having aheterocyclic segment which can be coordinated to a transition metalatom; each of X₀ and X independently represents a ligand which does nothave a heterocyclic segment; each of M₀ and M independently represents atransition metal atom; BL represents a bridging ligand having aplurality of portions, each of which can be coordinated to a transitionmetal atom; l1 is an integer of from 1 to 7 and l2 is an integer of from0 to 6, with the proviso that the sum of l1 and l2 is not more than 7; mis an integer of from 1 to 7, with the proviso that the sum of l1, l2and m is not more than 8; n1 is an integer of from 0 to 6 and n2 is aninteger of from 0 to 7, with the proviso that the sum of n1 and n2 isnot more than 7; and each of p and q independently represents an integerof 1or more,

wherein when each of l1, l2, m, n1, n2, pm and q is an integer of 2 ormore, the plurality of L₀ may be the same or different, the plurality ofX₀ may be the same or different, the plurality of BL may be the same ordifferent, the plurality of M may be the same or different, theplurality of L may be the same or different, and the plurality of X maybe the same or different.

With respect to the above-mentioned electron transfer, explanation isgiven below with reference to FIGS. 4 and 5, taking as an example a casewhere there is used a multinuclear complex having a structurerepresented by formula (1). For the sake of simplicity, it is assumedthat each of m, p and q in formula (1) is 1.

FIG. 4 is a diagrammatic view of a preferred example (described below)of the structure of a multinuclear complex represented by formula (1).L₀ is a ligand which comprises a heterocyclic segment and anon-heterocyclic segment (having a functional group which bonds to then-type semiconductor). Ligand L₀ is coordinated to metal atom M₀ througha hetero atom in the heterocyclic segment. BL is a bridging ligandhaving an asymmetric structure, which comprises a heterocyclic segmentand a non-heterocyclic segment. Bridging ligand BL is coordinated tometal atom M through a hetero atom in the heterocyclic segment, and isalso coordinated to metal atom M₀ through an atom in thenon-heterocyclic segment. As mentioned above, the probability of anelectron transition from a metal atom to a heterocyclic segment (i.e.,“metal to ligand charge transfer” (MLCT)) is high. Therefore, both ofthe electron transition from metal atom M₀ to ligand L₀ and the electrontransition from metal atom M to bridging ligand BL occur with highprobability.

By virtue of the asymmetric structure of bridging ligand BL, the flow ofelectrons in the multinuclear complex can be controlled in the directionindicated by symbol “(D)” in FIG. 5. Especially when the probability ofelectron transition from the occupied electron orbital ascribed to metalatom M to the unoccupied electron orbital ascribed to bridging ligand BL(i.e., probability of the electron transition indicated by symbol “(C)”)is higher than the probability of electron transition from the occupiedelectron orbital ascribed to metal atom M₀ to the unoccupied electronorbital ascribed to bridging ligand BL (i.e., probability of an electrontransition which occurs in the direction opposite to the direction ofthe electron transfer indicated by symbol “(D)”), the electrontransitions and electron transfers indicated in FIG. 5 occurefficiently. For this reason, it is preferred that bridging ligand BL iscoordinated to metal atom M through a heteroatom in the heterocyclicsegment, as diagrammatically indicated in FIG. 4.

Hereinbelow, detailed explanation is given with respect to the preferredstructure of the multinuclear complex represented by formula (1).

With respect to the multinuclear complex represented by formula (1), itis preferred that bridging ligand BL comprises a heterocyclic segmentand a non-heterocyclic segment, wherein the non-heterocyclic segment iscoordinated to metal atom M₀. Further, when q is an integer of 2 ormore, it is preferred that the heterocyclic segment of BL is coordinatedto the metal atom M which is close to metal atom M₀, and that a heteroatom in the heterocyclic segment is coordinated to at least one metalatom M. When the multinuclear complex represented by formula (1) has theabove-mentioned structure, an excited electron from metal atom M istransferred through bridging ligand BL to metal atom M₀ and, hence, itbecomes possible to efficiently transfer an electron excited from a lowenergy level to a component dye which has turned into anelectron-deficient state by an electron excitation.

In the present invention, the term “non-heterocyclic segment” means asegment which does not have a heterocycle and which is bonded to aheterocyclic segment. Examples of non-heterocyclic segments includeatoms (including those which are in the form of an ion, such as anoxoanion) and groups which do not have any heterocycles.

The structure of the multinuclear complex can be analyzed by any ofvarious analysis methods described below, and the type of metal atom M₀can be determined from the analysis. Specifically, the type of metalatom M₀ can be determined as follows. The structure of the multinuclearcomplex is analyzed, and the structure in which bridge ligand BL iscoordinated to the transition metal atoms is determined from theanalysis. Then, with respect to the transition metal atoms which arecoordinated to bridging ligand BL, the type of the transition metal atomcoordinated to the heterocyclic segment and the type of the transitionmetal atom coordinated to the non-heterocyclic segment are determined,and the transition metal atom coordinated to the non-heterocyclicsegment is designated as M₀.

With respect to the terminal component dyes of the multinuclear complexrepresented by formula (1), when one of the terminal component dyes doesnot have a heterocyclic segment other than the heterocyclic segment ofbridging ligand BL (i.e., when the terminal component dye(s) present onthe right-hand side of formula (1) does not have ligand L), thisterminal component dye is considered to be the component dye which ispositioned remote from the n-type semiconductor. Alternatively, in thecase where each of the terminal component dyes has a heterocyclicsegment other than the heterocyclic segment of bridging ligand BL (i.e.,the case where the terminal component dye(s) present on the right-handside of formula (1) has ligand L), when the heterocyclic segment of oneof the terminal component dyes has the below-mentioned functional groupwhich bonds to an n-type semiconductor or very small particles thereof(i.e., when the heterocyclic segment of L₀ or L has the below-mentionedfunctional group), the terminal component dye having such heterocyclicsegment is considered to be the component dye which is secured to then-type semiconductor.

With respect to the multinuclear complex represented by formula (1),when q is an integer of 2 or more, there is a bridging ligand BL whichis not directly coordinated to metal atom M₀, and such bridging ligandBL is coordinated through a non-heterocyclic segment to a metal atom Mwhich is close to metal atom M₀. The “metal atom which is close to metalatom M₀” is determined by the order in which the transition metal atoms(M₀ and M) and the bridging ligands (BL) are bonded. For example, when qis 2 and m is 1, the order in which the transition metal atoms (M₀ and aplurality of M) and the bridging ligands (BL) are bonded is shown by theformula: M₀(BL)(M)[BL][M]. In this case, the metal atom M which ispresent on the left-hand side of the formula (i.e., the metal atomrepresented by symbol “(M)”) is the “metal atom which is close to metalatom M₀” as compared to the metal atom M represented by the formula [M].

Hereinbelow, more detailed explanation is given with respect to thestructure of the multinuclear complex represented by formula (1).

With respect to the multinuclear complex represented by formula (1), theheterocyclic segment of bridging ligand BL may be a monodentate segmentor a multidentate segment having a dentate number of 2 or more. However,it is preferred that the heterocyclic segment of bridging ligand BL is amultidentate segment having a dentate number of 2 or more. When theheterocyclic segment of bridging ligand BL is a multidentate segmenthaving a dentate number of 2 or more, the coordinate bond becomesstrong. The upper limit of the dentate number of the heterocyclicsegment of bridging ligand BL may be appropriately selected depending onthe coordinate number of metal atom M; however, it is preferred that thedentate number is from 2 to 4.

In the multinuclear complex represented by formula (1), the heterocyclicsegment of bridging ligand BL coordinated to the metal atom M which isremote from the n-type semiconductor is a heterocyclic segment comprisedof at least one 5-membered, 6-membered or 7-membered ring which has atleast one heteroatom selected from the group consisting of a nitrogenatom, a sulfur atom and an oxygen atom. The heterocyclic segment ofbridging ligand BL is coordinated through such a heteroatom to the metalatom M which is remote from the n-type semiconductor. It is preferredthat the heterocyclic segment is comprised of two or more of 5-memberedrings, 6-membered rings or 7-membered rings which are chemically bondedto each other because the energy level of the heterocyclic segmentbecomes low. Further, it is preferred that the two or more of 5-memberedrings, 6-membered rings or 7-membered rings, each of which has at leastone heteroatom selected from the group consisting of nitrogen atom,sulfur atom and oxygen atom, have a structure as shown in FIG. 6,wherein the position of the heteroatoms (indicated by symbols “Za” and“Zb”) in the rings are diagrammatically shown. The reason for this isthat, when the heterocyclic segment has such a structure, the bondingbetween the heterocyclic segment and metal atom M becomes stronger.

Examples of heterocyclic segments include 5-membered ring compounds,6-membered ring compounds and 7-membered ring compounds, such aspyridine, pyrrole, pyran, furan, thiopyran, thiophene, pyrimidine,pyrazine, pyridazine, imidazole, triazole, pyrazole, thiazole, oxazole,caprolactum and caprolactone; and ring compounds in which theabove-mentioned ring compounds are chemically bonded, such asbipyridine, terpyridine, phenanthroline and a quaternary pyridine.

Among the above-mentioned ring compounds, especially preferred arebipyridine, terpyridine, phenanthroline, quaternary pyridine, andderivatives of these compounds, which are obtained by bonding thebelow-mentioned functional groups to the compounds.

The heterocyclic segment of bridging ligand BL has bonded thereto anon-heterocyclic segment which is coordinated to metal atom M₀. Theheterocyclic segment may have a functional group which can control theenergy level of the heterocyclic segment and/or improve the solubilityof the multinuclear complex in a solvent. Examples of such functionalgroups include a methyl group, an ethyl group, an n-propyl group, anisopropyl group, a t-butyl group, a phenyl group, a benzyl group, acarboxylic acid group, a sulfonic acid group, a phosphoric acid group, ahydroxyl group, a chloro group and a fluoro group. Further examples ofsuch functional groups include a phenyl group having bonded thereto acarboxyl group or the like. These functional groups can be appropriatelyused individually or in combination, depending on various properties ofthe functional groups, such as the compatibility with a solvent, theelectron-attracting property, and the electron-donating property. Byadjusting the resonance ratio of such an electron-attracting group orelectron-donating group to the heterocyclic segment, it becomes possibleto adjust the energy level of bridging ligand BL, thereby rendering itpossible to adjust the excitation levels of the component dyes which arechemically bonded to each other, and the energy levels of the componentdyes in excited states. Especially, the energy level of bridging ligandBL in an excited state has great influence on the transition width(i.e., absorption wavelength) of an electron from metal atom M which iscoordinated to bridging ligand BL on the side thereof remote from then-type semiconductor. Therefore, such adjustment of the energy level ofbridging ligand BL is important.

When bridging ligand BL has the above-mentioned functional group, it ispreferred that the functional group is different from thebelow-mentioned functional group of ligand L₀. The reason for this isthat, when the multinuclear complex is bonded to the n-typesemiconductor through the functional group of ligand L₀, it becomes easyto control the portion at which the multinuclear complex is bonded tothe n-type semiconductor.

In the multinuclear complex represented by formula (1), the“non-heterocyclic segment coordinated to the M₀” means a segment whichdoes not have a heterocycle and which is bonded to a heterocyclicsegment. The non-heterocyclic segment may be a functional group bondedto the heterocyclic segment, such as a hydroxyl group or an oxoanion(O⁻) which is obtained by ionizing a hydroxyl group. Alternatively, thenon-heterocyclic segment may comprise at least one atom bonded to theheterocyclic segment, and an atom or group bonded to the at least oneatom. With respect to the type of the atom contained in thenon-heterocyclic segment which is coordinated to M₀, there is noparticular limitation; however, it is preferred that the atom isselected from the group consisting of a nitrogen atom, an oxygen atom, asulfur atom and a hydrogen atom. Specific examples of non-heterocyclicsegments include a carbonyl group, an amino group, a thiol group, ahydroxyl group and an oxoanion (O⁻). As the non-heterocyclic segment,from the viewpoint of improving the strength of the linkage, preferredis a multidentate ligand having a dentate number of 2 or more, such as aβ-diketonato group, which has 2 carbonyl groups. Further, an ionizeddiol which has 2 oxoanions (O⁻) is also preferred from the viewpoint ofthe strength of linkage. Specifically, such an ionized diol can form anionic bond with the metal to which the bridging ligand is coordinated,depending on the type of the metal and the type of the ligand (otherthan bridging ligand BL) which is coordinated to the metal, so that thelinkage is strengthened.

Next, explanation is given with respect to preferred structures ofbridging ligand BL, referring to some specific examples. Each of FIG. 7(a) and FIG. 7( b) shows a bridging ligand which is derived from1,10-phenanthroline-5,6-dione (FIG. 7( c); hereinafter referred to as“quinone form”). Depending on the reaction atmosphere, the structure of1,10-phenanthroline-5,6-dione changes to a monovalent anion structure(hereinafter referred to as “semiquinone form”) as shown in FIG. 7( b)or a divalent anion structure (hereinafter referred to as “catecholanion form”) as shown in FIG. 7( a). Therefore, when1,10-phenanthroline-5,6-dione is used as bridging ligand BL, the1,10-phenanthroline-5,6-dione is often contained in the multinuclearcomplex in at least two forms selected from the group consisting of thequinone form, the semiquinone form and the catechol anion form. Further,with respect to this bridging ligand, in each of FIG. 7( a), FIG. 7( b)and FIG. 7( c), the heterocyclic segment is positioned on the left-handside of the ligand, and the non-heterocyclic segment is positioned onthe right-hand side of the ligand. In the catechol anion form, thenon-heterocyclic segment has a structure comprising oxoanions (O⁻).

Each of FIG. 8( a) and FIG. 8( b) shows further examples of bridgingligand BL. FIG. 8( a) shows a compound obtained by modifying aphenanthroline ring at the fourth and seventh positions thereof, whereina β-diketonato group is introduced as a non-heterocyclic segment throughthe methylene chain at the fourth position and an aldehyde group isintroduced at the seventh position for the purpose of adjusting theenergy level of the unoccupied orbital of bridging ligand BL. FIG. 8( b)shows an example of bridging ligand BL wherein a phenyl group having acatechol anion structure is introduced to the heterocyclic segment. Withrespect to each of the compounds shown in FIG. 8( a) and FIG. 8( b), thestructure of the non-heterocyclic segment may be changed to a structurehaving an ion as explained above with reference to FIGS. 7( a), 7(b) and7(c).

In formula (1), each of M₀ and M independently represents a transitionmetal atom. A transition metal is an element belonging to any of groups3 to 11 of the Periodic Table. Each of M₀ and M may be appropriatelyselected depending on the desired use; however, preferred is atransition metal selected from the group consisting of Fe, Ru, Os, Co,Rh, Ir, Ni, Pd, Pt, Cr, Mn, Cu, Mo, W and Re, and more preferred is atransition metal selected from the group consisting of Fe, Ru, Os, Co,Rh, Ir, Ni, Pd and Pt.

With respect to M₀, the use of an element which is widely used ispreferred for obtaining a complex which has a high probability oftransition to a heterocyclic segment in the photoelectric conversionelement or a dye-sensitized solar battery using the photoelectricconversion element. Specifically, as M₀, it is preferred to use atransition metal selected from the group consisting of Fe, Ru and Os,and it is most preferred to use Ru.

Further, it is preferred that M₀ is a hexacoordinate transition metal.The reason for this is as follows. As described below, in the field ofdye-sensitized solar battery, from the viewpoint of stability and energylevel, it is preferred to use, as ligand L₀, a multidentate heterocycliccomplex having a dentate number of from 2 to 4, such as bipyridine,phenanthroline, terpyridine, quaternary pyridine and derivatives ofthese compounds. Therefore, a hexacoordinate transition metal whichcorresponds to such a multidentate compound can be preferably used asM₀.

On the other hand, it is preferred that M is a tetracoordinate orhexacoordinate transition metal. The coordinate number of M isdetermined taking into consideration the adjustment of the energy levelof each of the component dyes; the number of the portions (i.e., Land/or X in formula (1)) to which electrons are introduced from anelectrolyte when the multinuclear complex is used in a dye-sensitizedsolar battery; the stability of the structure of the multinuclearcomplex; and the ease in the production of the multinuclear complex. M₀and M may be the same or different (a plurality of M may be present inthe multinuclear complex represented by formula (1)); however, theoxidation potentials of M₀ and M are different. Specifically, theoxidation potential of M₀ is higher than the or each M (that is, thevalue of the oxidation of M₀ is smaller than that of the or each M),and, when a plurality of M are present, the plurality of M are arrangedin an order such that the nearer (to M₀) the M, the higher the oxidationpotential of the M. By arranging M₀ and M in the above-mentioned order,it becomes possible to more efficiently take out excited electrons fromorbitals having low energy levels. The oxidation potentials of M₀ and Mcan be measured by an electrochemical method (e.g., cyclic voltammetry),wherein the measurement is performed with respect to M₀ and Mcoordinated to a ligand.

The probability of electron transition varies greatly depending on thetypes of M₀ and M, as well as the type of the ligand (i.e., L₀ informula (1)) positioned on the n-type semiconductor side of M₀ and M andthe type of the bridging ligand (i.e., BL in formula (1)). Therefore, asdescribed above in connection with the composite dye, for transferringelectrons in the composite dye in a desired direction, it is preferredto choose a combination of a transition metal and a ligand, whichachieves a high probability of electron transition.

In formula (1), each of L₀ and L independently represents a ligandhaving a heterocyclic segment which can be coordinated to a transitionmetal atom. The heterocyclic segment of L₀ or L is comprised of at leastone 5-membered ring, 6-membered ring or 7-membered ring which has atleast one heteroatom selected from the group consisting of a nitrogenatom, a sulfur atom and an oxygen atom. L₀ or L is coordinated throughthe at least one heteroatom to metal atom M₀ or M which is remote fromthe n-type semiconductor. It is preferred that the heterocyclic segmentis comprised of two or more 5-membered rings, 6-membered rings or7-membered rings, because the energy level of the heterocyclic segmentbecomes lowered. Further, as in the case of the above-mentioned bridgingligand BL, it is preferred that each of the two or more of 5-memberedrings, 6-membered rings or 7-membered rings which has at least oneheteroatom selected from the group consisting of nitrogen atom, sulfuratom and oxygen atom has a structure as shown in FIG. 6, because thebonding between the heterocyclic segment and M₀ and the bonding betweenthe heterocyclic segment and M become stronger.

Examples of such heterocyclic segments include 5-membered ringcompounds, 6-membered ring compounds and 7-membered ring compounds, suchas pyridine, pyrrole, pyran, furan, thiopyran, thiophene, pyrimidine,pyrazine, pyridazine, imidazole, triazole, pyrazole, thiazole, oxazole,caprolactam and caprolactone; and ring compounds in which theabove-mentioned ring compounds are chemically bonded, such asbipyridine, terpyridine, phenanthroline and quaternary pyridine.

The above-mentioned heterocyclic segment may have a functional group.Examples of functional groups include a methyl group, an ethyl group, ann-propyl group, an isopropyl group, a t-butyl group, a phenyl group, abenzyl group, a carboxylic acid group, a sulfonic acid group, aphosphoric acid group, a hydroxyl group, a chloro group and a fluorogroup. Further examples of functional groups include a phenyl grouphaving a carboxyl group bonded thereto. The above-mentioned functionalgroups can be appropriately used individually or in combination,depending on various properties of the functional groups, such aschemical bonding property, electron-attracting property andelectron-donating property.

It is preferred that L₀ is a bi- to quadridentate ligand. Specifically,as L₀, it is preferred to use a compound comprising a bi- toquadridentate ligand in which the above-mentioned 5-membered, 6-memberedor 7-membered rings are chemically bonded. Examples of such ligandsinclude bipyridine, terpyridine, phenanthroline and quaternary pyridine.Further, it is preferred that L₀ has a functional group which can bondto the n-type semiconductor or very small particles thereof.Specifically, it is preferred that L₀ has a functional group selectedfrom the group consisting of a carboxylic acid group, a carboxylic acidsalt group, a sulfonic acid group, a sulfonic acid salt group, aphosphoric acid group and a phosphoric acid salt group. Alternatively,L₀ may have a functional group which, when bonded to L₀, is capable offorming substantially the same bond as that of one of theabove-exemplified functional groups. Examples of such functional groupsinclude a carboxylic ester, a phosphoric ester and a sulfonic ester.Furthermore, when l1 is an integer of 2 or more, it is preferred that atleast one L₀ has the above-mentioned functional group. In this case,when a plurality of L₀ have two or more of the above-mentionedfunctional groups, these functional groups may be the same or different.

As a more preferred example of L₀, there can be mentioned a ligandhaving a heterocycle, which is generally used as a sensitizer for aphotoelectric conversion element. As such a ligand, there can be used abi- to quadridentate ligand which is comprised of a conjugatedheterocycle having a nitrogen atom and a functional group capable ofbonding to the n-type semiconductor. Specific examples of such ligandsinclude bipyridine, terpyridine, phenanthroline and quaternary pyridine,each of which has bonded thereto a carboxylic acid group, a carboxylicacid salt group, a phosphoric acid group or a phosphoric acid saltgroup.

On the other hand, it is preferred that L is a ligand having a mono- totridentate heterocycle which can be coordinated to a transition metalatom.

L may or may not be used. L mainly has a function of adjusting theenergy level of metal atom M in a ground state. When the multinuclearcomplex is used in a dye-sensitized solar battery, L also has a functionof receiving an electron from an electrolyte. When L has a functionalgroup, it is preferred that the functional group of L is different fromthat of L₀. The reason for this is that, when the multinuclear complexrepresented by formula (1) is bonded to the n-type semiconductor or verysmall particles thereof through the functional group of L₀, it becomeseasy to control the portion at which the multinuclear complex is bondedto the n-type semiconductor or very small particles thereof.

In formula (1), each of X₀ and X independently represents a ligand whichdoes not have a heterocycle. As each of X₀ and X, it is preferred to usean anionic or neutral ligand. Specific examples of X₀ and X includeligands in the form of ions or molecules, such as a fluoro ligand, achloro ligand, a bromo ligand, a hydroxo ligand, a cyano ligand, athiocyanato ligand, an isothiocyanato ligand, a nitro ligand, acarbonato ligand, a phosphato ligand, a sulfito ligand, a sulfatoligand, an acyloxy ligand, an acylthio ligand, an ammine ligand,ethylenediamine and derivatives thereof, triethylenetetramine andderivatives thereof, a carbonyl ligand and a phosphine ligand. Furtherexamples of X₀ and X include aliphatic or aromatic compounds andderivatives thereof, which have bonded thereto any of theabove-mentioned ligands (e.g., a carbonyl group, an amino group, a thiolgroup, a hydroxyl group and an ion thereof).

In formula (1) above, l1 is an integer of from 1 to 7 and l2 is aninteger of from 0 to 6, with the proviso that the sum of l1 and l2 isnot more than 7. l1 and l2 are determined depending on the coordinationnumber of transition metal atom M₀ and the structure of the portion atwhich bridging ligand BL is coordinated to M₀. l1 is preferably aninteger of from 1 to 3. As already mentioned above, it is preferred thatL₀ is a bi- to quadridentate ligand; thus, l1 is appropriatelydetermined in accordance with the coordination number of M₀. On theother hand, l2 is preferably an integer of from 0 to 2. The reason forthis is that X₀ is used mainly as a supplementary ligand when thecoordination number of M₀ is not fulfilled, depending on thecoordination number of M₀ and the coordination structure of each of L₀and the non-heterocyclic segment of BL.

Preferred examples of combinations of l1 and l2 include the followingcases (A) to (D):

(A) a case where M₀ is a hexacoordinate metal atom, L₀ is a bidentateligand (e.g., bipyridine or a derivative thereof, or phenanthroline or aderivative thereof), l1 is 2, the portion at which BL is coordinated toM₀ is bidentate (e.g., a β-diketonato group), and l2 is 0;

(B) a case where M₀ is a hexacoordinate metal atom, L₀ is a tridentateligand (e.g., terpyridine or a derivative thereof), l1 is 1, the portionat which BL is coordinated to M₀ is tridentate, and l2 is 0;

(C) a case where M₀ is a hexacoordinate metal atom, L₀ is a tridentateligand (e.g., terpyridine or a derivative thereof), l1 is 1, the portionat which BL is coordinated to M₀ is bidentate, and l2 is 1; and

(D) a case where M₀ is a hexacoordinate metal atom, L₀ is aquadridentate ligand (e.g., quaternary pyridine or a derivativethereof), l1 is 1, the portion at which BL is coordinated to M₀ isbidentate, and l2 is 0.

As mentioned above, M₀ is preferably a hexacoordinate metal; thus, asexemplified above, it is more preferred that l1 is 1 or 2 and that l2 is0 or 1.

In formula (1) above, m is an integer of from 1 to 7, with the provisothat the sum of l1, l2 and m is not more than 8. The number m isdetermined depending on the coordination number of M₀ and thecoordination structures of L₀, X₀, and the portion at which BL iscoordinated to M₀. When m is 2 or more, the multinuclear complex has abranched structure such that a plurality of portions each containing abridging ligand BL branch out from M₀. For preventing the structure ofthe multinuclear complex from being so complicated, it is preferred thatm is an integer of from 1 to 3, more advantageously 1 or 2, still moreadvantageously 1. Especially, when the non-heterocyclic segment of BL isbidentate, m is preferably 1.

As mentioned above, M₀ is preferably a hexacoordinate metal. When M₀ isa hexacoordinate metal, the sum of l1, l2 and m is 5 or less.

In formula (1) above, n1 is an integer of from 0 to 6 and n2 is aninteger of from 1 to 7, with the proviso that the sum of n1 and n2 isnot more than 7. The numbers n1 and n2 are determined depending on thecoordination number of M and the structure of the portion at whichbridging ligand BL is coordinated to M.

Preferred examples of combinations of n1 and n2 include the followingcases a) to l):

a) a case where M is a tetracoordinate metal, BL is coordinated to Mthrough a bidentate ligand (e.g., bipyridine or a derivative thereof, orphenanthroline or a derivative thereof) as the heterocyclic segmentthereof, n1 is 0, X is a monodentate ligand, and n2 is 2;

b) a case where M is a tetracoordinate metal, BL is coordinated to Mthrough a bidentate ligand (e.g., bipyridine or a derivative thereof, orphenanthroline or a derivative thereof) as the heterocyclic segmentthereof, n1 is 0, X is a bidentate ligand, and n2 is 1;

c) a case where M is a tetracoordinate metal, BL is coordinated to Mthrough a tridentate ligand (e.g., terpyridine or a derivative thereof)as the heterocyclic segment thereof, n1 is 0, X is a monodentate ligand,and n2 is 1;

d) a case where M is a hexacoordinate metal, BL is coordinated to Mthrough a bidentate ligand (e.g., bipyridine or a derivative thereof, orphenanthroline or a derivative thereof) as the heterocyclic segmentthereof, n1 is 0, X is a monodentate ligand, and n2 is 4;

e) a case where M is a hexacoordinate metal, BL is coordinated to Mthrough a bidentate ligand (e.g., bipyridine or a derivative thereof, orphenanthroline or a derivative thereof) as the heterocyclic segmentthereof, n1 is 0, X is a bidentate ligand, and n2 is 2;

f) a case where M is a hexacoordinate metal, BL is coordinated to Mthrough a bidentate ligand (e.g., bipyridine or a derivative thereof, orphenanthroline or a derivative thereof) as the heterocyclic segmentthereof, L is a bidentate ligand, n1 is 1, X is a monodentate ligand,and n2 is 2;

g) a case where M is a hexacoordinate metal, BL is coordinated to Mthrough a bidentate ligand (e.g., bipyridine or a derivative thereof, orphenanthroline or a derivative thereof) as the heterocyclic segmentthereof, L is a bidentate ligand, n1 is 1, X is a bidentate ligand, andn2 is 1;

h) a case where M is a hexacoordinate metal, BL is coordinated to Mthrough a bidentate ligand (e.g., bipyridine or a derivative thereof, orphenanthroline or a derivative thereof) as the heterocyclic segmentthereof, L is a bidentate ligand, n1 is 2, and n2 is 0;

i) a case where M is a hexacoordinate metal, BL is coordinated to Mthrough a tridentate ligand (e.g., terpyridine or a derivative thereof)as the heterocyclic segment thereof, n1 is 0, X is a monodentate ligand,and n2 is 3;

j) a case where M is a hexacoordinate metal, BL is coordinated to Mthrough a tridentate ligand (e.g., terpyridine or a derivative thereof)as the heterocyclic segment thereof, n1 is 0, X is a tridentate ligandand, n2 is 1;

k) a case where M is a hexacoordinate metal, BL is coordinated to Mthrough a quadridentate ligand (e.g., quaternary pyridine or aderivative thereof) as the heterocyclic segment thereof, n1 is 0, X is amonodentate ligand, and n2 is 2; and

l) a case where M is a hexacoordinate metal, BL is coordinated to Mthrough a quadridentate ligand (e.g., quaternary pyridine or aderivative thereof) as the heterocyclic segment thereof, n1 is 0, X is abidentate ligand, and n2 is 1.

In formula (1) above, p is an integer of 1 or more, and is determineddepending on the coordinate structure of the heterocyclic segmentthrough which BL is coordinated to M, and the number of ligandscontained in the heterocyclic segment. For example, when theheterocyclic segment has one bidentate ligand (e.g., bipyridine or aderivative thereof, or phenanthroline or a derivative thereof) or onetridentate ligand (e.g., terpyridine or a derivative thereof), p is 1;when the heterocyclic segment has two bidentate ligands (whereinexamples of such ligands include bipyridine, a derivative of bipyridine,phenanthroline and a derivative of phenanthroline), p is 1 or 2,preferably 2; and when the heterocyclic segment has three monodentateligands (wherein examples of such ligands include quinoline and aderivative thereof), p is an integer of from 1 to 3, preferably 3. Withrespect to the upper limit of p, there is no particular limitation.However, when p becomes large, the structure of the multinuclear complexbecomes so complicated that it becomes difficult to produce themultinuclear complex. Therefore, it is preferred that p is an integer offrom 1 to 3, more advantageously 1 or 2. When p is 1, the structure ofthe multinuclear complex becomes most simple.

In formula (1) above, q is an integer of 1 or more, which represents thenumber of recurrence of [(BL)_(m){M(L)_(n1)(X)_(n2)}_(pm)] units. When qis 2 or more, the multinuclear complex becomes a complex having 3 ormore nuclei. In such a case, a plurality of BL are directionallypositioned on both sides of each of a plurality of M in a manner suchthat the non-heterocyclic segments thereof are located on the near sideof M₀, so that the multinuclear complex contains a linear structureBL-M-BL.

With respect to the multinuclear complex represented by formula (1), asq becomes large, it becomes possible to absorb light ray having a widerlight wavelength distribution, and to obtain a higher voltage by usingan appropriate electrolyte (described below). However, when q is 3 ormore, a disadvantage is caused wherein the structure of the multinuclearcomplex becomes complicated, so that it becomes difficult to producesuch a multinuclear complex. This disadvantage cannot be counterbalancedby the above-mentioned advantage of absorbing light ray having a widerlight wavelength distribution and obtaining a higher voltage. Therefore,it is preferred that q is 1 or 2. Further, from the viewpoint of ease inthe production of the multinuclear complex, it is more preferred that qis 1.

When the multinuclear complex represented by formula (1) above has anelectric charge, a counter ion may be used to neutralize the electriccharge. Examples of counter ions include anions, such as atetrafluoroboron ion, a tetrafluorophosphorus ion, a perchloric acidion, a chloride ion, a bromine ion, an iodine ion, a nitric acid ion, asulfuric acid ion, an isothiocyanic acid ion and a thiocyanic acid ion;a hydrogen ion; alkali metal ions and alkaline earth metal ions, such asa lithium ion, a sodium ion, a potassium ion, a cesium ion, a magnesiumion, a calcium ion, a strontium ion and a barium ion; an ammonium ion;organic ammonium ions, such as a tetramethylammonium ion, atetraethylammonium ion, a monomethylammonium ion, a dimethylammoniumion, a trimethylammonium ion, a tetrabutylammonium ion and atriphenylammonium ion; and organic phosphonium ions, such as atetraphenylphosphonium ion.

Next, explanation is given with respect to the method for synthesizingthe above-mentioned multinuclear complex. The multinuclear complex canbe synthesized by performing desired ligand exchange reactions incombination, taking into consideration the bonding ability of eachligand to each metal atom. Specifically, there can be mentioned thefollowing methods:

(1) a method in which a complex represented by the formula(L₀)_(l1)(X₀)_(l2)M₀ and a complex represented by the formula(BL)_(m){M(L)_(n1)(X)_(n2)}_(pm) are separately synthesized, and(BL)_(m){M(L)_(n1)(X)_(n2)}_(pm) is bonded to (L₀)_(l1)(X₀)_(l2)M₀,wherein the reaction for forming this bonding may involve a ligandexchange reaction;(2) a method in which a complex represented by the formula(L₀)_(l1)(X₀)_(l2)M₀(BL)_(m) is synthesized, and M(L)_(n1)(X)_(n2) isbonded thereto;(3) a method in which a complex represented by the formula(L₀)_(l1)(X₀)_(l2)M₀(BL)_(m) is synthesized, and a salt of M is bondedthereto, followed by bonding of (L)_(n1)(X)_(n2); and(4) a method in which (L₀)_(l1)(X₀)_(l2)M₀, M(L)_(n1)(X)_(n2) and BL aremixed and bonded to each other.

In the above-mentioned method (1), the complex represented by theformula (L₀)_(l1)(X₀)_(l2)M₀ can be synthesized, for example, by addingL₀ and a chloride salt of M₀ to a solvent, and heating the resultantmixture. In this case, the amount of L₀ to be added to the solvent isdetermined from the coordination numbers of M₀, L₀ and X₀ (wherein thechloride ion which derives from the chloride salt of M₀ corresponds toX₀ which is to be left in the produced complex represented by theformula (L₀)_(l1)(X₀)_(l2)M₀). For example, in the case where L₀ is abidentate ligand (e.g., 4,4′-dicarboxy-2,2′-bipyridine) and M₀ isruthenium (which is a hexacoordinate metal), when ruthenium chloride isused as a raw material and it is intended to leave two chloride ions inthe produced complex (represented by the formula (L₀)_(l1)(X₀)_(l2)M₀)per molecule thereof, L₀ is used in an amount of two times the molaramount of M₀. The reaction for obtaining (L₀)_(l1)(X₀)_(l2)M₀ ispreferably performed in a solvent (e.g., dimethylformamide) which cansatisfactorily solve L₀ and the chloride salt of M₀. This reaction isgenerally performed at a stable reflux temperature for several hours.The compound obtained by this reaction contains chloride ion(s) as X₀which is to be replaced by (BL)_(m){M(L)_(n1)(X)_(n2)}_(pm) in thesubsequent ligand exchange reaction.

With respect to the complex represented by the formula(BL)_(m){M(L)_(n1)(X)_(n2)}_(pm), the method for synthesis thereof isdescribed below, taking as an example a case where m=p=1 and, hence,(BL)_(m){M(L)_(n1)(X)_(n2)}_(pm) is (BL)M(L)_(n1)(X)_(n2). Also in thiscase, it is preferred to use, as a raw material, a chloride salt oracetate salt of M which is susceptible to a ligand exchange reaction.Further, when it is difficult to coordinate BL or L directly to thechloride salt or acetate salt of M, it is preferred that adimethylsulfoxide (DMSO) complex or the like is first produced as anintermediate, and the above-mentioned ligands (i.e., BL and L) aresuccessively coordinated to M while heating and stirring. For example,when it is intended to synthesizediisothiocyanato-(1,10-phenanthroline-5,6-dione)platinum(II), thesynthesis thereof can be performed as follows. Potassiumtetrachloroplatinate(II) is dissolved in a solvent, such as water, andthe resultant is mixed with DMSO in an amount of 2 to 4 times the molaramount of the potassium tetrachloroplatinate(II) to thereby form a DMSOcomplex of platinum chloride. Then, the DMSO complex is mixed with1,10-phenanthroline-5,6-dione which has a coordination ability strongerthan that of DMSO (wherein the amount of the1,10-phenanthroline-5,6-dione is equimolar to the amount of the DMSOcomplex), to thereby replace DMSO by 1,10-phenanthroline-5,6-dione, sothat 1,10-phenanthroline-5,6-dione is coordinated to platinum throughthe nitrogen atoms in the phenanthroline ring thereof. Finally, thechloride ions remaining on the platinum atom are replaced byisothiocyanate ions (an isothiocyanate ion has a coordination abilitystronger than that of a chloride ion). Since a chloride ion can beeasily replaced by another ion, it is preferred that chloride ions areleft on M until the final ligand exchange reaction for obtaining thedesired complex is performed.

After obtaining the complexes represented by the formulae(L₀)_(l1)(X₀)_(l2)M₀ and (BL)_(m){M(L)_(n1)(X)_(n2)}_(pm), a finalligand exchange reaction is performed for obtaining the multinuclearcomplex. In the above-mentioned case (in this case,(BL)_(m){M(L)_(n1)(X)_(n2)}_(pm) is (BL)M(L)_(n1)(X)_(n2)),(BL)M(L)_(n1)(X)_(n2) is replaced by a part of or all of X₀ in(L₀)_(l1)(X₀)_(l2)M₀, so that (BL)M(L)_(n1)(X)_(n2) is coordinated to(L₀)_(l1)(X₀)_(l2)M₀ through the dione portion of1,10-phenanthroline-5,6-dione. The dione structure (i.e., theabove-mentioned quinone form) sometimes lowers the reaction rate;therefore, it is preferred to perform the reaction while forming theabove-mentioned catechol anion by using a solvent (e.g.,dimethylformamide) or basic compound (e.g., potassium hydroxide) whichcan moderately reduce the dione structure to the catechol anion.

Further, although a DMSO complex is mentioned above as an example of theabove-mentioned intermediate, a nitrile (e.g., acetonitrile orbenzonitrile), a cyclooctadiene ring, triphenyl phosphine or an aceticion can also be preferably used for forming such an intermediate.

In the above-mentioned method (2), firstly, the complex represented bythe formula (L₀)_(l1)(X₀)_(l2)M₀(BL)_(m) is synthesized. When BL has anasymmetric structure, which is preferred in the present invention, it isnecessary that the direction in which BL is coordinated to M₀ beadjusted. For example, in the case where BL is5,6-dihydroxy-1,10-phenanthroline, when it is intended to coordinate BLto M₀ through the oxoanion structure, a basic compound, such as sodiumhydroxide or potassium hydroxide, is added to the solvent so as toperform the synthesis of (L₀)_(l1)(X₀)_(l2)M₀(BL)_(m) under basicconditions. By performing the synthesis of (L₀)_(l1)(X₀)_(l2)M₀(BL)_(m)under basic conditions, the coordination of the oxoanion can be causedto occur preferentially over the coordination of the nitrogen atoms onthe phenanthroline ring, so that the direction in which BL iscoordinated to M₀ can be adjusted.

The synthesis of (L₀)_(l1)(X₀)_(l2)M₀(BL)_(m) also involves a ligandexchange reaction in which X₀ is replaced by BL, and it is preferredthat X₀ is a chloride ion. For example, when it is intended to react(cis-dichloro-bis(2,2′-bipyridyl-4,4′-dicarboxylic acid))rurthenium(II)with 5,6-dihydroxy-1,10-phenanthroline while ionizing the hydroxyl groupof the 5,6-dihydroxy-1,10-phenanthroline to an oxoanion, a mixed solventof dimethylformamide and water (wherein the mixed solvent satisfactorilydissolves the solute) is used and potassium hydroxide is added theretoso that the reaction can be performed under basic conditions, and thereaction is performed, while heating, under reflux in an inert gasatmosphere (e.g., a nitrogen or argon atmosphere) for several hours. Bythis reaction, the chloride ions of(cis-dichloro-bis(2,2′-bipyridyl-4,4′-dicarboxylic acid))rurthenium(II)are replaced by ionized 5,6-dihydroxy-1,10-phenanthroline to therebyobtain (L₀)_(l1)(X₀)_(l2)M₀(BL)_(m).

To the obtained (L₀)_(l1)(X₀)_(l2)M₀(BL)_(m) is bondedM(L)_(n1)(X)_(n2), which has been separately synthesized.M(L)_(n1)(X)_(n2) can be synthesized in substantially the same manner asin the synthesis of (L₀)_(l1)(X₀)_(l2)M₀ in the above-mentioned method(1). As explained above in connection with method (1), when it isdifficult to coordinate M to the heteroatom of the heterocycle of BL,which heterocycle has a conjugated double bond, the synthesis isperformed after an intermediate, such as a DMSO complex, is produced. Inthe synthesis of a complex, strong coordination bond is sometimespreferentially formed. In such a case, after the desired compound hasbeen formed, an undesirable ligand exchange reaction may occur, so thatthe structure of the desired compound is changed. In this case, it ispreferred that the reaction time is not longer than required. Forexample, when the synthesis of a chloride salt of(bis(2,2′-bipyridyl-4,4′-dicarboxylicacid)(1,10-phenanthroline-5,6-diolate))rurthenium(II)-(bis(2,2′-bipyridyl))rurthenium(II)is performed in a mixed solvent of water and ethanol at a refluxtemperature, the reaction time is preferably about 1 hour. Further, whenthe complexation constant can be calculated and the activation energycan be determined therefrom, the reaction temperature may be elevated orlowered so as to suppress the above-mentioned undesirable ligandexchange reaction.

In method (3), the complex represented by the formula(L₀)_(l1)(X₀)_(l2)M₀(BL)_(m) is synthesized in substantially the samemanner as in the above-mentioned method (2), and a chloride salt oracetate salt of M is bonded thereto, followed by bonding of (L)_(n1)and/or (X)_(n2). In this method, when the chloride salt or acetate saltof M is bonded to (L₀)_(l1)(X₀)_(l2)M₀(BL)_(m), there is a possibilitythat the desired compound which has already been synthesized is bondedto M so as to form a symmetric dimer in which M is the center ofsymmetry. Therefore, it is preferred that the dimerization of thedesired compound is suppressed by lowering the concentration of the rawmaterials and the reaction temperature, and that a purification isperformed after the reaction to thereby recover the desired compound.

In method (4), it is preferred that BL has a symmetric structure, sincethe desired complex can be easily obtained. Further, even a BL having anasymmetric structure can also be used when the coordination ability ofM₀ is different from that of M. Specifically, for example, in the casewhere BL is 5,6-dihydroxy-1,10-phenanthroline or a derivative thereof,when the coordination of oxoanion to M₀ occurs faster than thecoordination of the nitrogen atoms of phenanthroline to M₀, and thecoordination of the nitrogen atoms of phenanthroline to M occurs fasterthan the coordination of oxoanion to M, BL can be coordinated to M₀ andM in the desired direction.

Each of the components of the multinuclear complex is synthesized asfollows.

Firstly, as mentioned above, the bonding abilities of the ligands to themetal atoms are compared with each other. This comparison is performedby reacting each ligand with a precursor of a salt of each metal, withreference to reaction rate and examples of synthesis of similarcompounds which are described in the literature. It is preferred toperform the above-mentioned comparison prior to the synthesis of themultinuclear complex, because it becomes easy to determine the structureof the multinuclear complex by the below-mentioned analysis.Subsequently, the synthesis of the multinuclear complex is performed ina designed order. The synthesis is performed in a specific solvent(hereinafter referred to as “synthesis solvent”) which can dissolve orsuspend the starting material. Examples of synthesis solvent include anorganic solvent and water. These solvents can be used individually or incombination. The synthesis solvent may contain an additive, such as anorganic or inorganic salt. Depending on the type of the desired linkage,various properties (e.g., acidity or basicity and oxidizing property orreducing property) of the synthesis solvent may play an important rolein the progress of the reaction and the stability of the product.Therefore, the additive is selected taking into consideration not onlythe solubility thereof in the synthesis solvent but also theabove-mentioned properties.

The reaction temperature and reaction time for the synthesis reactionare determined, taking into consideration the boiling point of theorganic solvent used (when an organic solvent is used) and the ease inperforming the preliminary experiment mentioned above for determiningthe reaction order. It is preferred that the synthesis reaction istraced by various analysis methods described below to thereby determinethe endpoint of the synthesis reaction.

The synthesis reaction is generally performed under atmosphericpressure. Further, in many cases, it is preferred that an inert gas(e.g., nitrogen or argon) is blown into the solvent during the synthesisreaction.

When the synthesis solvent is a poor solvent for the reaction product,the reaction product may precipitate in the synthesis solvent, so thatthe reaction product can be easily separated therefrom. Alternatively,the reaction product can also be obtained by a method in which a part ofthe synthesis solvent is removed by distillation to thereby precipitatethe reaction product. In such a case, the reaction product is recoveredfrom the synthesis solvent by a separation method, such as filtrationusing a filter, or centrifugal separation. Alternatively, if desired,the reaction product can be recovered by a method in which the synthesissolvent is completely distilled off to obtain a solid substance (i.e., amixture comprising the desired compound and raw materials) and theobtained solid substance is washed with a poor solvent for the desiredcompound; or by a purification method, such as a recrystallizationmethod, a reprecipitation method, a method in which liquids areseparated according to their types, or a column purification methodusing an adsorbent (e.g., silica, alumina or an organic silica). Thenecessity of the purification and the type of the purification methodare determined, taking into consideration the properties of the reactionproduct, and the facility and economical efficiency of the purificationmethod.

The structure of the multinuclear complex is determined by infrared (IR)spectroscopy, nuclear magnetic resonance (NMR) spectroscopy,ultraviolet-visible (UV-vis) spectroscopy, mass spectroscopy (MS), ICPemission spectroscopy, fluorescent X-ray spectroscopy, a combination ofvarious chemical, elemental analysis methods, or a method in which asingle crystal is formed and the interatomic distance is determined byX-ray diffractometry.

As a composite dye, there can be mentioned a composite dye which isobtained by chemically bonding various organic dyes having a π-conjugatebond and optionally having a bonding group introduced thereinto.Examples of such organic dyes include a 9-phenylxanthene dye, atriphenylmethane dye, an acridine dye, a coumarin dye, an indigoid dye,a cyanin dye, a spiropyran dye, an azo dye and a xanthene dye. Thesynthesis for obtaining such a composite dye can be performed by any ofthe conventional organic synthesis methods which are mentioned above inconnection with the synthesis of the multinuclear complex. Further, theanalysis of such a composite dye can be performed by any of the analysismethods which are mentioned above in connection with the synthesis ofthe multinuclear complex. Specific examples of the above-mentionedorganic dyes are described in “JOEM Handbook 2: Absorption Spectra ofDyes for Diode Lasers”, written by Ken Matsuoka and published byBun-Shin Shuppan, Japan (1990).

In the present invention, with respect to the composite dye comprising aplurality of component dyes which have different excitation levels andwhich are chemically bonded to each other, the wavelength ranges oflight absorbed by the component dyes may be the same or different;however, it is preferred that the wavelength ranges are different. Whenthe wavelength ranges of light absorbed by the component dyes aredifferent, it becomes possible to use light ray (such as sunray) havinga wide distribution of light wavelengths.

Hereinbelow, explanations are given with respect to the n-typesemiconductor used in the photoelectric conversion element of thepresent invention. In the present invention, the n-type semiconductorworks as follows. The n-type semiconductor receives an excited electronwhich is generated by the absorption of light ray by the composite dye,and transfers the excited electron to an electroconductive material usedfor collecting electrons.

Therefore, it is required that the conduction-band level of the n-typesemiconductor be lower than the energy level of the component dye(secured to the n-type semiconductor) in an excited state. It ispreferred that the conduction-band level of the n-type semiconductor islower than the energy level of the lowest unoccupied molecular orbital(LUMO) of the component dye secured to the n-type semiconductor.

Specific examples of n-type semiconductors include various oxides, suchas titanium oxide, tin oxide, zinc oxide, indium oxide, niobium oxide,tungsten oxide and vanadium oxide; various compound oxides, such asstrontium titanate, calcium titanate, barium titanate and potassiumniobate; cadmium sulfide; bismuth sulfide; cadmium selenide; cadmiumtelluride; gallium phosphide; and gallium arsenide. These semiconductorsmay be used individually or in combination. Of these semiconductors,titanium oxide is preferred since titanium oxide has a good balance ofthe capability for receiving excited electrons from the composite dyeand the capability for transferring the received electrons to atransparent electroconductive membrane.

With respect to the shape of the n-type semiconductor, there is noparticular limitation; however, it is preferred that the n-typesemiconductor is used in a particulate form. The reason for thepreference for a particulate n-type semiconductor is as follows. Aparticulate n-type semiconductor has an increased surface area forreceiving excited electrons from the composite dye, so that a largeamount of the composite dye can be efficiently used as a light absorber.Therefore, the photoelectric conversion efficiency can be improved.

The size of the n-type semiconductor particle varies depending on theintended use of the photoelectric conversion element, the intensity ofthe light ray irradiated to the composite dye, and the absorptioncapacity of the composite dye. It is preferred that the n-typesemiconductor particle has a primary particle size of from 1 to 5,000nm, more advantageously from 2 to 100 nm, most advantageously from 2 to50 nm. When the primary particle size is more than 5,000 nm, a problemis likely to arise in that the light transmission properties of then-type semiconductor as a membrane are lowered and, hence, the incidentlight cannot be fully utilized. On the other hand, when the primaryparticle size is less than 1 nm, a problem is likely to arise in thatthe electron conductivity of the particulate semiconductor is loweredand, hence, a great loss of electrons may occur during the transfer ofexcited electrons from the composite dye to the below-mentionedelectroconductive support.

The particle size of the particulate n-type semiconductor can bemeasured by using, for example, a laser scattering particle sizedistribution analyzer or a dynamic light scattering spectrophotometer.Alternatively, the particle size of the particulate n-type semiconductorcan also be measured by a method using a scanning electron microscope,in which a photomicrograph taken with respect to the photoelectricconversion element containing the n-type semiconductor is used todetermine the particle size of the n-type semiconductor particle. Inthis method, the particle size is measured with respect to all particlesshown in the photomicrograph and, then, the average value of theparticle sizes is defined as the particle size of the n-typesemiconductor particle. With respect to each particle, when the particleis spherical, the particle size means the diameter of the particle; and,when the particle is non-spherical, the particle size means the averagevalue of the lengths of the longest and shortest sides of the particle.

The n-type semiconductor may have, in the surface thereof, a shell layercapable of adjusting the electron conductivity. By virtue of such ashell layer, in some case, it is possible to suppress the occurrence ofa flowback of electrons to the composite dye having emitted excitedelectrons or to a material (e.g., electrolyte) other than the n-typesemiconductor and the composite dye. In the present invention, when theflowback of excited electrons (transferred from the composite dye) tothe composite dye does not easily occur, the above-mentioned stepwisemultiphoton absorption process works more efficiently. Therefore,especially when the n-type semiconductor is made of a highlyelectroconductive material (e.g., tin oxide or zinc oxide), the shelllayer may play an important role.

As a material for the shell layer, there can be mentioned a materialused for the n-type semiconductor or an insulator. Specific examples ofmaterials for the shell layer include various oxides, such as titaniumoxide, tin oxide, zinc oxide, indium oxide, niobium oxide, tungstenoxide and vanadium oxide; various compound oxides, such as strontiumtitanate, calcium titanate, magnesium titanate, barium titanate andpotassium niobate; inorganic n-type semiconductors, such as cadmiumsulfide, bismuth sulfide, cadmium selenide, cadmium telluride, galliumphosphide and gallium arsenide; alkali metal carbonates, such as lithiumcarbonate, sodium carbonate and potassium carbonate; alkaline earthmetal carbonates, such as magnesium carbonate, calcium carbonate,strontium carbonate and barium carbonate; transition metal carbonates,such as cobalt carbonate, nickel carbonate and manganese carbonate;metal carbonates, such as lanthanoid carbonates (e.g., lanthanumcarbonate, ytterbium carbonate and cerium carbonate); alkali metaloxides, such as lithium oxide, sodium oxide and potassium oxide;alkaline earth metal oxides, such as magnesium oxide, calcium oxide,strontium oxide and barium oxide; transition metal oxides, such ascobalt oxide and manganese oxide; metal oxides, such as aluminum oxide,lanthanoid oxides (e.g., cerium oxide, gadolinium oxide, samarium oxideand ytterbium oxide); inorganic insulators, such as natural andsynthetic silicates (e.g., silica); and low molecular weight and highmolecular weight organic insulators. These materials can be usedindividually or in combination. Of these materials, from the viewpointof the stability of the materials, preferred is an inorganic compoundselected from the group consisting of an inorganic n-type semiconductorand an inorganic insulator, and more preferred is an inorganic compoundcontaining an alkaline earth metal.

With respect to the thickness of the shell layer, there is a freedom ofchoice. However, from the viewpoint of assuring the probability that thetransfer of excited electrons from the composite dye to the n-typesemiconductor occurs, it is preferred that the thickness of the shelllayer is less than 1 nm, more advantageously 0.8 nm or less, still moreadvantageously 0.6 nm or less, most advantageously 0.4 nm or less. Withrespect to the lower limit of the thickness of the shell layer, there isno particular limitation so long as there can be achieved an increase inthe open-circuit voltage of a dye-sensitized solar battery using thephotoelectric conversion element; however, it is preferred that thethickness of the shell layer is 0.1 nm or more.

The thickness of the shell layer can be measured by a visual observationusing a transmission electron microscope (TEM). Alternatively, thethickness of the shell layer can be measured by the X-ray photoelectronspectroscopy in which the profiling depth is generally 5 nm or less. Inthe X-ray photoelectron spectroscopy, the thickness of the shell layeris calculated from the atomic ratio between a specific element of then-type semiconductor (e.g., titanium when the n-type semiconductor ismade of titanium oxide) and a specific element of the shell layer (e.g.,calcium when the shell layer is made of calcium carbonate), and thespecific gravity of the shell layer, which is known from the compositionof the shell layer determined by the below-mentioned method. Withrespect to the specific elements used for calculating the atomic ratio,the elements are appropriately selected in view of the ease in theanalysis. Specifically, the elements are such that there are fewoverlapping peaks and such that the intensity of the peaks is high. Itis preferred to use elements which are contained in only one of then-type semiconductor and the shell layer component.

Further, the thickness of the shell layer can also be measured by amethod in which the compositions of the shell layer and the n-typesemiconductor are determined using an instrument (e.g., a time-of-flightsecondary ion mass spectrometer (TOM-SIMS)) while performing an etchingof the shell layer. Specifically, etching of the shell layer isperformed until the change of the composition is elicited, and thethickness of the shell layer is obtained as the depth of etching whenthe change of the composition is elicited. Moreover, the thickness ofthe shell layer can be more concisely determined by a calculation usingthe values of the specific gravity and amount of each of the materialsused in the shell layer and/or the n-type semiconductor, and the averageparticle size of the n-type semiconductor (when the n-type semiconductoris particulate).

As explained hereinabove, the photoelectric conversion element of thepresent invention comprises a composite dye and an n-type semiconductor.The n-type semiconductor may be used in a form such that the n-typesemiconductor is in contact with another material, e.g., anelectroconductive material mentioned below. Specifically, when aparticulate n-type semiconductor is used, from the viewpoint offacilitating the electron flow in the n-type semiconductor, it ispreferred that the n-type semiconductor is present, in the form of amembrane having a porous structure, on the surface of theelectroconductive material, wherein the n-type semiconductor particlesare sintered. The term “porous structure” used herein is defined asfollows. The surface area per weight of a portion of the semiconductormembrane is measured by, e.g., BET adsorption isotherm using a nitrogengas as an adsorbent. When the thus measured surface area per weight isat least 5 times, preferably at least 10 times, more preferably at least50 times, as large as a planar area per weight of the portion of thesemiconductor (wherein the planar area is obtained by projecting theportion on a plane), it is defined that the semiconductor membrane has aporous structure. Such an n-type semiconductor in the form of a membranehaving a porous structure can be used in combination with theabove-mentioned n-type semiconductor having a shell layer.

The semiconductor membrane may further contain an additive in an amountsuch that the below-mentioned properties of the photoelectric conversionelement are not deteriorated. Examples of additives include organicbinders (e.g., acetylacetone), metal peroxides (e.g., titanium peroxide,tin peroxide and niobium peroxide), inorganic binders (e.g., metalalkoxides), inorganic compounds (e.g., nitric acid and sulfuric acid),polymeric compounds (e.g., polyethylene glycol, polypropylene glycol,cellulose and derivatives thereof), surfactants (e.g., nonionicsurfactants, anionic surfactants, cationic surfactants and siliconesurfactants) and chelating agents.

Examples of electroconductive materials to be in contact with the n-typesemiconductor include metals (such as gold, silver, copper, platinum andpalladium) and thin films thereof; and transparent electroconductivematerials, such as indium oxide compounds (e.g., tin-doped indium oxide,i.e., indium tin oxide (ITO)), tin oxide compounds (e.g., fluorine-dopedtin oxide (FTO)), zinc oxide compounds, and thin films thereof.

Instead of the composite dye comprising a plurality of component dyeswhich have different excitation levels and which are chemically bondedto each other, the photoelectric conversion element of the presentinvention may contain a dye selected from the group consisting of acomplex dye and an organic dye, each of which has excitation levelscomparable to those of the component dyes used in the present invention.

In the present invention, the amount (amount carried on the n-typesemiconductor) of each of the composite dye and the above-mentionedcomplex dye and organic dye can be determined by an ultraviolet-visiblespectroscopy in which the absorbance of the dye is measured and thefound absorbance is converted into the amount of the dye. Themeasurement of the amount of the dye can be performed with respect to abattery containing the photoelectric conversion element. Alternatively,the measurement of the amount of the dye can be performed with respectto the dye which has been separated from the n-type semiconductor byusing an aqueous alkali solution.

Hereinbelow, explanations are given with respect to the dye-sensitizedsolar battery of the present invention. The dye-sensitized solar batteryof the present invention comprises an electrode (i.e., a photo-anode)comprised of the photoelectric conversion element of the presentinvention, a counter electrode, and an electrolyte interposed betweenthe photoelectric conversion element and the counter electrode, whereinthe dye-sensitized solar battery becomes operable when the electrodecomprised of the photoelectric conversion element and the counterelectrode are connected to each other through an electroconductivematerial which is positioned outside of the electrolyte.

In the dye-sensitized solar battery of the present invention, it ispreferred that at least one of the two electrodes contains a transparentelectroconductive support comprising a transparent substrate havingsupported thereon a transparent electroconductive material, wherein thetransparent electroconductive material may be used in the form of a thinfilm. Specific examples of transparent electroconductive materialsinclude indium oxide compounds (e.g., tin-doped indium oxide, i.e.,indium tin oxide (ITO)), tin oxide compounds (e.g., fluorine-doped tinoxide (FTO)) and zinc oxide compounds.

In the dye-sensitized solar battery of the present invention, the twoelectrodes (i.e., the photo-anode and the counter electrode) and theelectrolyte may be so arranged that the electrolyte is sandwichedbetween the two electrodes (hereinafter, such a dye-sensitized solarbattery is referred to as “sandwich-type battery”). Alternatively, thetwo electrodes and the electrolyte may be so arranged that the twoelectrodes are immersed in the electrolyte (hereinafter, such adye-sensitized solar battery is referred to as “immersion-typebattery”).

As mentioned above, the photoelectric conversion element of the presentinvention comprises a composite dye and an n-type semiconductor, whereinthe composite dye is generally carried on the n-type semiconductor. Then-type semiconductor works as follows: the n-type semiconductor receivesan excited electron which is generated by the absorption of light ray bythe composite dye, and transfers the excited electron to anelectroconductive material used for collecting electrons. Therefore, then-type semiconductor is in contact with an electroconductive material.That is, a layer comprising the composite dye and the n-typesemiconductor is formed on the surface of the electroconductivematerial, and the electroconductive material having on the surfacethereof a layer comprising the composite dye and the n-typesemiconductor functions as a photo-anode (i.e., photoelectric conversionelectrode).

In the present invention, it is preferred that the electroconductivematerial for the photo-anode is transparent. The reason for this is asfollows. In the dye-sensitized solar battery of the present invention, atransparent electroconductive support is used for introducing light.Therefore, when the photoelectric conversion element is positioned inthe light-introducing side, there is an advantage in that loss of theintroduced light due to the absorption thereof by the electrolyte doesnot occur and, hence, the energy of the introduced light can beeffectively utilized. An example of such preferred structure of thedye-sensitized solar battery of the present invention isdiagrammatically shown in FIG. 9.

For preventing the occurrence of the leakage of electrons from thetransparent electroconductive material to the electrolyte, thetransparent electroconductive material may have, in the surface thereof,a layer for preventing a flowback of electrons, so long as the transferof electrons from the n-type semiconductor to the transparentelectroconductive material is not markedly disturbed. As a material forthe layer for preventing a flowback of electrons, it is preferred to usea titanium oxide which has a low crystallinity or is amorphous. Thelayer for preventing a flowback of electrons can be formed by thesol-gel method or the sputtering method.

In the present invention, the term “transparent” means that the lighttransmission is 10% or more. With respect to a transparent material, thelight transmission thereof is preferably 50% or more, more preferably70% or more.

With respect to the material used as the transparent substrate, there isno particular limitation, and there can be used a glass and an organicsubstance each having the above-defined transparency. Specific examplesof organic substances include transparent polymer films, such aspolyethylene terephthalate (PET), polyethylene naphthalate (PEN),syndiotactic polystyrene (SPS), polyphenylene sulfide (PPS), polyarylate(PAr), polysulfone (PSF), polyethersulfone (PES), polyetherimide (PEI),polycarbonate (PC), tetraacetyl cellulose (TAC) and poly(methylmethacrylate) (PMMA).

In the dye-sensitized solar battery of the present invention, theelectrolyte has a function of supplying electrons to the composite dyewhich has turned into an electron-deficient state after donating, to then-type semiconductor, excited electrons generated by the irradiation ofthe composite dye with light ray. As the electrolyte, there is used anelectrolyte which has a potential suitable for the redox potential ofthe composite dye. When the electrolyte has such a potential, electronscan be transferred from the electrolyte to the composite dye, wherebythe dye-sensitized solar battery of the present invention works. Theexpression “a potential suitable for the redox potential of thecomposite dye” means an energy level higher than that of the componentdye positioned most remote from the n-type semiconductor (i.e., thecomponent dye having the lowest excitation level); in other words, theabove expression means a potential value smaller than that of thecomponent dye, wherein the potential value is measured by anelectrochemical method. The use of the electrolyte having such an energylevel is advantageous in that the transfer of electrons from theelectrolyte to the composite dye is facilitated.

The suitability of the potential of the electrolyte can be determined byan electrochemical method, such as cyclic voltammetry.

In the present invention, the term “the potential of the counterelectrode” (wherein the counter electrode is positioned opposite to thephoto-anode comprised of the photoelectric conversion element) means theenergy level of the counter electrode, at which electrons aretransferred from the counter electrode to the electrolyte. At thisenergy level, the electrolyte in an oxidized form is reduced, and theresultant electrolyte in a reduced form can transfer electrons therefromto the composite dye in the photoelectric conversion element. Thepotential of the counter electrode can be determined by the followingphotoelectrochemical method using a three-electrode typephotoelectrochemical measurement and a two-electrode typephotoelectrochemical measurement. In the three-electrode typephotoelectrochemical measurement, there are used a photo-anode comprisedof the photoelectric conversion element (i.e., photoelectric conversionelectrode) as a working electrode, a counter electrode (e.g., platinum)and a reference electrode which is used as a reference potential. On theother hand, in the two-electrode type photoelectrochemical measurement,there are used only a working electrode (which is the same as above) anda counter electrode, and the voltage between the counter electrode andthe working electrode is measured. From the two values of voltageobtained by the measurements, the potential of the counter electrode isobtained as the electric potential of the counter electrode relative tothe redox potential of the reference electrode. Alternatively, thepotential of the counter electrode can also be determined by athree-electrode type electrochemical measurement in which the currentand voltage are measured without irradiation with light ray, underconditions wherein no oxidation wave or reduction wave can be detected(such conditions can be realized, for example, by satisfactorilylowering the voltage scanning rate or stirring the sample liquidcontaining the electrolyte. In this method, a current-voltage curve isobtained. The intersection point between the current-voltage curve andthe voltage axis (generally, the x-axis), at which no oxidation currentor reduction current can be detected, is determined as the potential ofthe counter electrode.

The voltage generated in the dye-sensitized solar battery of the presentinvention is determined by the potential difference between thephoto-anode comprised of the photoelectric conversion element (i.e.,photoelectric conversion electrode) as a working electrode, and thecounter electrode. Therefore, from the viewpoint of yielding a highvoltage of the dye-sensitized solar battery, it is preferred that thecounter electrode has a low potential (that is, the counter electrodehas a large redox potential value as measured by the electrochemicalmethod).

In the present invention, with respect to the minimum potential of thecounter electrode (i.e., the maximum redox potential as measured by theelectrochemical method), there is no particular limitation so long asthe minimum potential is higher than the energy level of the componentdye positioned most remote from the n-type semiconductor (i.e., thecomponent dye having the lowest excitation level) (in this case, thedye-sensitized solar battery of the present invention works). However,it is preferred that the minimum potential of the counter electrode ishigher than the energy level of the component dye having the lowestexcitation level by 0.05 V or more, more advantageously 0.1 V or more.On the other hand, with respect to the maximum potential of the counterelectrode, there is no particular limitation so long as the maximumpotential is lower than the conduction-band level of the n-typesemiconductor (in this case, the dye-sensitized solar battery of thepresent invention works). However, from the viewpoint of yielding a highvoltage of the dye-sensitized solar battery, it is preferred that themaximum potential of the counter electrode is in the range such that thedifference between the energy level of the component dye having thelowest excitation level and the maximum potential is 1 V or less, moreadvantageously 0.8 V or less, still more advantageously 0.5 V or less,most advantageously 0.3 V or less.

The composite dye used in the present invention comprises a plurality ofcomponent dyes which have different excitation levels and which arechemically bonded to each other, wherein the plurality of component dyesare arranged in an order such that the excitation levels of theplurality of component dyes are decreased as viewed from the n-typesemiconductor. It is preferred that the composite dye has a structuresuch that excited electrons in each component dye move directionallytoward the n-type semiconductor. By making an advantage of the electrontransfer from the component dye positioned most remote from the n-typesemiconductor, it becomes possible to transfer electrons (which aretaken from the electrolyte at a low energy level) step by step to then-type semiconductor. By virtue of such mechanism, it becomes possibleto achieve a potential difference between the photo-anode (comprised ofthe photoelectric conversion element) and the counter electrode, whereinthe potential difference is larger than the potential differencecorresponding to the electron transition width (corresponding to theabsorption spectrum) of each component dye. Therefore, for achieving apotential difference between the photo-anode and the counter electrodeby the above-mentioned mechanism, it is preferred that the counterelectrode exhibits a potential as low as possible, so long as thepotential is higher than the energy level of the component dye havingthe lowest excitation level. More specifically, it is preferred that thecounter electrode exhibits a potential of −0.2 V or more, moreadvantageously 0 V or more, still more advantageously 0.3 V or more,most advantageously 0.5 V or more, relative to the redox potential ofsilver/silver ion as measured in accordance with the electrochemicalmethod. Especially, when the counter electrode exhibits a potentiallower than the excitation level of the component dye secured to then-type semiconductor (i.e., first photoabsorption portion), there is anadvantage in that the composite dye can receive, from the electrolyte,an electron having a low energy level which cannot be received by thefirst photoabsorption portion alone, thereby achieving a potentialdifference which is larger than that corresponding to the electrontransition width in the first photoabsorption portion. By using suchelectrode and such counter electrode in combination under conditionswherein the counter electrode exhibits a potential larger than thatcorresponding to the electron transition width in the component dyesecured to the n-type semiconductor (i.e., first photoabsorptionportion), it becomes possible to operate a dye-sensitized solar batteryhaving a high energy conversion efficiency, in which the above-mentionedtheoretical limit of the one-photon absorption process is overcome.

Specific examples of redox couples include a combination of iodine andan iodide (e.g., lithium iodide, tetrabutyl ammonium iodide ortetrapropyl ammonium iodide); a combination of bromine and a bromide; acombination of chlorine and a chloride; a combination of an alkylviologen and a reduced form thereof; a combination of quinone andhydroquinone; a combination of nonequivalent ions of a transition metal,such as a combination of iron(II) ion and iron(III) ion, a combinationof copper(I) ion and copper(II) ion, a combination of manganese(II) ionand manganese(III) ion, and a combination of cobalt(II) ion andcobalt(III) ion; a combination of compounds containing complex ions,such as a combination of ferrocyanide and a ferricyanide, a combinationof cobalt(II) tetrachloride and cobalt(III) tetrachloride, a combinationof cobalt(II) tetrabromide and cobalt(III) tetrabromide, a combinationof iridium(II) hexachloride and iridium(III) hexachloride, a combinationof ruthenium(II) cyanide and ruthenium(III) cyanide, a combination ofrhodium(II) hexachloride and rhodium(III) hexachloride, a combination ofrhenium(III) hexachloride and rhenium(IV) hexachloride, a combination ofrhenium(IV) hexachloride and rhenium(V) hexachloride, a combination ofosmium(III) hexachloride and osmium(IV) hexachloride, and a combinationof osmium(IV) hexachloride and osmium(V) hexachloride; a complex formedbetween a transition metal (such as cobalt, iron, ruthenium, manganese,nickel or rhenium) and a conjugated heterocyclic compound or aderivative thereof (such as bipyridine, a derivative of bipyridine,terpyridine, a derivative of terpyridine, phenanthroline, or aderivative of phenanthroline); a complex formed between a metal and acyclopentadiene or a derivative thereof (such as a ferrocene/ferroceniumion, cobaltocene/cobaltcenium ion, or a ruthenocene/ruthenoseum ion);and a porphyrin compound.

Of these redox couples, from the viewpoint of yielding a high voltage,preferred are those which enable the counter electrode to exhibit apotential in the above-mentioned preferred range.

The electrolyte used in the dye-sensitized solar battery of the presentinvention may be or may not be in the form of a solution thereof. Whenthe electrolyte is used in the form of a solution thereof, theelectrolyte solution may contain an oxidizing agent or a reducing agentfor adjusting the potential of the counter electrode. The reason is asfollows. Most electrolytes can have a plurality of valences. Therefore,by using an electrolyte solution containing an oxidizing agent or areducing agent, there can be obtained a redox couple having a desiredvalence, which can be advantageously used for receiving and donatingelectrons in the dye-sensitized solar battery (i.e., for receivingelectrons from the counter electrode and donating electrons to thecomposite dye which has turned into an electron-deficient state). Such amethod for adjusting the potential of the counter electrode by the useof an oxidizing agent or a reducing agent is advantageous especiallywhen the electrolyte contains a transition metal and, hence, is likelyto have a plurality of valences.

The oxidizing agent and reducing agent can be appropriately selected,depending on the redox potential of the electrolyte. Representativeexamples of oxidizing agents include nitrosonium boron tetrafluoride.Representative examples of reducing agents include organic or inorganicsulfinic acids and salts thereof, and ascorbic acids and salts thereof.

When it is desired to enhance the electric conductivity of theelectrolyte solution, the electrolyte solution may further contain asupporting electrolyte. Specific examples of supporting electrolytesinclude organic or inorganic perchlorates, such as lithium perchlorate,sodium perchlorate, ammonium perchlorate, tetramethylammoniumperchlorate, tetraethylammonium perchlorate and tetrabutylammoniumperchlorate; and organic or inorganic hexafluorophosphates, such aslithium hexafluorophosphate, sodium hexafluorophosphate, ammoniumhexafluorophosphate, tetramethylammonium hexafluorophosphate,tetraethylammonium hexafluorophosphate and tetrabutylammoniumhexafluorophosphate. With respect to these supporting electrolytes, itis preferred to use, as an organic solvent therefor, aprotic polarsolvents. Specific examples of aprotic polar solvents includeacetonitrile, methoxyacetonitrile, methoxypropionitrile, ethylenecarbonate, propylene carbonate, dimethylformamide, dimethyl sulfoxide,1,3-dimethylimidazolidinone and 3-methyloxazolidinone.

In the dye-sensitized solar battery of the present invention, the redoxcouple works as an electron carrier. Therefore, it is necessary that theredox couple be contained in the electrolyte with a certain level ofconcentration. Specifically, the concentration of the redox couple inthe electrolyte is preferably 0.001 mol/l or more, more preferably 0.01mol/l or more, still more preferably 0.3 mol/l or more. With respect tothe upper limit of the concentration, there is no particular limitation.However, in the case where a colored electrolyte is used, there is aproblem in that the composite dye is shielded from light ray by thecolored electrolyte and, hence, the amount of light absorbed by thecomposite dye is likely to be decreased. In view of this, it ispreferred that the concentration of the redox couple in the electrolyteis 3 mol/l or less. Further, when the concentration of the oxidant inthe electrolyte is too high, a problem is likely to arise in that aflowback of electrons from the photoelectric conversion element to theelectrolyte is caused to occur. Therefore, the concentration ratiobetween the reductant and oxidant in the electrolyte is appropriatelyselected by evaluating the photoelectric conversion performance of thedye-sensitized solar battery. In general, it is preferred that theconcentration of the reductant is higher than that of the oxidant. Evenwhen only one of an oxidant and a reductant is used as the electrolytein the dye-sensitized solar battery, it is possible that the electrolytereceives electrons from the counter electrode or donates electrons tothe composite dye which has turned into an electron-deficient state. Asa result, either an oxidant or a reductant (whichever has not been usedas the electrolyte) is generated, so that the electrolyte eventuallycontains both an oxidant and a reductant and, thus, the dye-sensitizedsolar battery works. For example, in the case where only a reductant isused as the electrolyte, when the dye-sensitized solar battery isirradiated with light rays, the composite dye is likely to emitelectrons and the resultant composite dye in an electron-deficient statedeprives the reductant of electrons, thereby causing the reductant to beconverted into an oxidant. Thus, the resultant electrolyte eventuallycontains both an oxidant and a reductant, wherein the concentrationratio between the oxidant and the reductant generally falls within theabove-mentioned preferred range such that the concentration of thereductant is higher than that of the oxidant.

The concentration of the electrolyte in the electrolyte solution variesdepending on the chemical structure of the electrolyte and the type ofthe solvent used. When the above-mentioned electrolyte is used, it ispreferred that the electrolyte is subjected, prior to use thereof, to areaction to form an organic salt (e.g., an organic ammonium salt)thereof, or caused to have bonded thereto a functional group having ahigh affinity to the solvent.

As mentioned hereinabove, the electrolyte is generally used in the formof a solution thereof in an organic solvent. However, mainly forpreventing the leakage of the electrolyte, the electrolyte can also beused either in the form of the so-called “gel electrolyte” comprising apolymer matrix impregnated with a solution of the electrolyte in anorganic solvent, or in the form of a molten salt thereof. The polymermatrix may be prepared by a method in which a polymerization isperformed in an organic solvent containing a redox couple, or may beused in the form of a sheet. Preferred examples of polymer matrixes inthe form of a sheet include a polyolefin or cellulose microporousmembrane used as a separator for a lithium ion battery or a condenser, ablood separation membrane and a moistening membrane. With respect to thethickness of the microporous membrane, it is preferred that thethickness is small. In general, a microporous membrane having athickness of from 2 to 20 μm is used. With respect to the porosity ofthe microporous membrane, it is preferred that the microporous membranehas as many pores as possible, from the viewpoint of the diffusionefficiency of the redox couple. The preferred porosity of themicroporous membrane also varies depending on the strength of themembrane. However, the porosity of the microporous membrane, in terms of% by volume of pores, based on the volume of the microporous membrane,is generally in the range of from 30 to 90%, preferably from 50 to 90%.

Further, the electrolyte can also be used in the form of an organic orinorganic solid electrolyte (i.e., a p-type semiconductor). Specificexamples of solid electrolytes include organic polymers having ahole-transporting property; and p-type semiconductors such as CuI, CuSCNand NiO.

Hereinbelow, explanations are given with respect to the method forproducing the photoelectric conversion element of the present inventionand the method for producing the dye-sensitized solar battery of thepresent invention. With respect to these methods, there is no particularlimitation, and any conventional methods can be employed.

A representative example of the method for producing the dye-sensitizedsolar battery is explained below. In this method, first, an electrodecomprised of a photoelectric conversion element and an electroconductivesupport is produced by a process comprising the steps of: preparing aliquid dispersion of a particulate n-type semiconductor, applying theprepared liquid dispersion onto the surface of an electroconductivesupport, followed by sintering, to thereby form a semiconductor membraneon the surface of the electroconductive support, and causing theobtained semiconductor membrane to adsorb a photosensitizer containing acomposite dye so that the photosensitizer is secured to (or carried on)the n-type semiconductor, thereby obtaining an electrode (i.e.,photoelectric conversion electrode) comprised of a photoelectricconversion element and an electroconductive support. Then, anotherelectrode (i.e., counter electrode) and a layer comprising anelectrolyte are provided opposite to the photoelectric conversionelectrode. Then, a layer comprising an electrolyte is interposed betweenthe photoelectric conversion electrode and the counter electrode, or anelectrolyte is introduced into the space between the two electrodes, tothereby obtain a dye-sensitized solar battery. If desired, thedye-sensitized solar battery can be sealed so as to prevent the leakageof the electrolyte.

With respect to the dispersion medium used in the liquid dispersion ofthe particulate n-type semiconductor, there is no particular limitationso long as the dispersion medium can maintain a liquid state at roomtemperature. Specific examples of dispersion mediums include water;alcohols, such as ethanol, methanol, propanol, butanol and isopropylalcohol; hydrophilic organic solvents, such as acetone, acetonitrile,propionitrile, dimethyl sulfoxide and dimethyl formamide; hydrophobicorganic solvents, such as dichloromethane, chloroform, carbontetrachloride, ethyl acetate, diethyl ether, tetrahydrofuran andtoluene; and a mixture thereof.

When it is desired to improve the dispersibility of n-type semiconductorparticles in the dispersion medium or to adjust the viscosity of thedispersion, the dispersion may further contain an additive. Examples ofadditives include organic binders (e.g., acetylacetone); inorganicbinders, such as metal peroxides (e.g., titanium peroxide, tin peroxideand niobium peroxide) and metal alkoxides; inorganic compounds, such asnitric acid and sulfuric acid; polymers, such as polyethylene glycol,polypropylene glycol, cellulose and derivatives thereof; surfacesurfactants, such as nonionic surface surfactants, anionic surfacesurfactants, cationic surface surfactants and silicone surfacesurfactants; and auxiliary chelating agents. Further, if desired, anacidic compound or basic compound may be added to the dispersion forimproving the dispersibility of n-type semiconductor particles in thedispersion medium.

With respect to the concentration (in terms of weight percentage) of thesolid content in the liquid dispersion, there is no particularlimitation. The concentration of the solid content in the liquiddispersion can be appropriately selected, depending on the properties ofthe liquid dispersion with respect to, e.g., spreadability andfast-drying; however, it is preferred that the concentration is in therange of from 10 to 50%, more advantageously from 15 to 40%.

With respect to the conditions for dispersing the n-type semiconductorparticles in the dispersion medium, there is no particular limitation.When it is desired to improve the uniformity of the liquid dispersion,it is advantageous to use a mixing-agitating type blender or anultrasonic homogenizer, such as a paint shaker, a ball mill or ahomogenizer. It is also useful to pulverize the n-type semiconductorparticles by using a mortar or the like before the mixing thereof withthe dispersion medium.

With respect to the method for applying the liquid dispersion of n-typesemiconductor particles onto the surface of the electroconductivesupport, there is no particular limitation so long as a semiconductormembrane can be formed on the surface of the electroconductive support.For example, the liquid dispersion can be applied to the surface of theelectroconductive support by a screen printing method, a spin coatingmethod, a dip coating method, a doctor blade method, or a method using awire bar. If desired, after the application of the liquid dispersiononto the surface of the electroconductive support, a step of drying atroom temperature may be performed. In the case where the liquiddispersion is repeatedly applied to the surface of the electroconductivesupport, it is preferred that the above-mentioned drying step isperformed after each application. The drying step can be omitted byperforming the below-mentioned sintering step.

In the sintering step, the sintering temperature varies depending on thetype of semiconductor used, the degree of sintering desired, the heatresistance of the electroconductive support used, and the like.Therefore, the sintering temperature can be appropriately selected inaccordance with the desired effect. In general, it is preferred that thesintering is performed at a high temperature, since the particles can bewelded to each other in a short period of time, thus increasing theconductivity between the particles. However, the use of a hightemperature is inappropriate for substances which may cause a phasetransition accompanied by a change of the crystallinity thereof, leadingto a lowering of the photoelectric conversion performance of thephotoelectric conversion element.

The type of the electroconductive support used is another significantfactor for selecting the sintering temperature. It is because theelectroconductive support is comprised of a transparent substrate and atransparent electroconductive membrane, each having a unique maximumtolerable temperature. For example, when an organic substance (e.g., apolymer film) having a low melting temperature or softening point isused as a transparent substrate and ITO (indium tin oxide) is used as atransparent electroconductive membrane, the sintering step can beperformed at a temperature lower than the maximum tolerable temperatureof the organic substance (e.g., a polymer film). Specifically, thesintering step is performed preferably at 250° C. or lower, morepreferably at 200° C. or lower. On the other hand, when a glass is usedas a transparent substrate and FTO (fluorine-doped tin oxide) is used asa transparent electroconductive membrane, the sintering step needs to beperformed at a temperature lower than the maximum tolerable temperatureof the glass. Specifically, the sintering step is performed preferablyat 600° C. or less. The time for sintering is preferably from 10 minutesto 1 hour, more preferably from 20 minutes to 1 hour.

With respect to the atmosphere in which the sintering step is performed,there is no particular limitation, and the atmosphere can beappropriately selected in accordance with the desired effect. Specificexamples of atmospheres include an inert gas atmosphere, such as anitrogen gas atmosphere or an argon gas atmosphere; a reducing gasatmosphere, such as a hydrogen gas atmosphere; a mixed gas atmosphere,such as a nitrogen/oxygen mixed gas atmosphere; air; a carbon dioxidegas atmosphere; and an oxygen gas atmosphere.

With respect to the semiconductor membrane produced under theabove-mentioned conditions, the thickness thereof is preferably in therange of from 0.5 to 50 μm, more preferably from 1.0 to 30 μm. When thethickness of the semiconductor membrane is less than 0.5 μm, there is aproblem in that the membrane cannot adsorb a satisfactory amount ofphotosensitizer mentioned below and, hence, a high level of energyconversion efficiency is not likely to be attained. On the other hand,when the thickness of the semiconductor membrane is more than 50 μm,problems arise not only in that the semiconductor membrane has a poormechanical strength and, hence, the membrane is likely to peel off fromthe electroconductive support, but also in that the light permeabilityof the semiconductor membrane is lowered and, hence, the photosensitizerwhich is adsorbed by the semiconductor membrane cannot receive asatisfactory amount of light. Further, a problem is also likely to arisein that the route of the electron transfer in the semiconductor membranebecomes prolonged, thus increasing the internal resistance, leading to alowering of the energy conversion efficiency.

Hereinbelow, explanations are given with respect to the step of causingthe semiconductor membrane to adsorb the photosensitizer comprising thecomposite dye. First, a solvent for the composite dye is selected. Thesolvent is selected in view of the solubility of the composite dye inthe solvent. Specific examples of solvents include alcohols, such asmethanol, ethanol, propanol and butanol; ketones, such as acetone andmethyl ethyl ketone; esters, such as ethyl acetate and butyl acetate;sulfolane; tetrahydrofuran; dimethyl sulfoxide; dimethylformamide andmixtures thereof. The n-type semiconductor (in the form of a membrane)formed on the electroconductive support (wherein the n-typesemiconductor and the electroconductive support forms a photo-anodeprecursor) is contacted with a solution of the photosensitizercomprising the composite dye in the above-mentioned solvent so that thecomposite dye is carried on the n-type semiconductor, thereby obtainingan electrode (i.e., a photo-anode). This step may be performed at roomtemperature for a period of several hours to several days; however, thestep is preferably performed at a temperature at which the solvent isrefluxed, or at a temperature in the range of from 50° C. to atemperature near the reflux temperature. When the step is performedunder such temperature conditions, there are advantages not only in thatthe solubility of the composite dye in the solvent increases and, hence,the amount of composite dye carried on the n-type semiconductorincreases, but also in that, when the composite dye is chemically bondedto the n-type semiconductor, the bonding between the composite dye andthe n-type semiconductor is likely to become stronger. There is afurther advantage in that the time necessary for the n-typesemiconductor to adsorb the composite dye is reduced to as short as 10minutes to several hours, which is preferred from the viewpoint ofcommercial productivity.

As described hereinabove, in the photoelectric conversion element of thepresent invention comprising a composite dye and an n-typesemiconductor, the transfer of electrons between a plurality ofcomponent dyes bonded to each other is likely to occur with a highprobability, so that it becomes possible to use light ray having a widelight wavelength distribution for taking out electrons from thecomposite dye. Such an excellent effect of the present invention isapparent from the comparison of the results between Example 1 andComparative Example 1 and the comparison of the results between Example2 and Comparative Example 2 (these Examples and Comparative Examples aredescribed below in detail). Further, in the dye-sensitized solar batteryof the present invention, it is possible to receive electrons from theelectrolyte which has received electrons from the counter electrodehaving a low potential, so that the dye-sensitized solar battery canyield a high voltage. Such an excellent effect of the present inventionis apparent from the comparison of the results between Examples 3 to 6and Comparative Examples 3 to 4 (these Examples and Comparative Examplesare described below in detail). Moreover, a comparison of the resultsbetween Examples 7 to 8 and Reference Example 1 shows that a two-photonabsorption process works advantageously in the dye-sensitized solarbattery of the present invention containing the photoelectric conversionelement. Such stepwise multiphoton absorption process works moreadvantageously when strong light is used. Therefore, when the stepwisemultiphoton absorption process is used in the application field of anoptical memory, in which strong light is used to record information andweak light is used to read the recorded information, the deteriorationof the recorded information can be effectively suppressed. Also, thestepwise multiphoton absorption process is different from the one-photonabsorption process in the electron-generating properties relative to theintensity of light, and thus the dye-sensitized solar battery of thepresent invention can also be used in the fields of optical switchingand optical sensors.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinbelow, the present invention will be described in more detail withreference to the following Examples, Comparative Examples and ReferenceExample, which should not be construed as limiting the scope of thepresent invention.

The measurements performed in the present invention are as follows.

(1) Ultraviolet-Visible Spectrophotometry (UV-vis)

Ultraviolet-visible spectrophotometry is performed with respect to awavelength range of from 300 nm to 800 nm using an ultraviolet-visiblespectrophotometer (trade name: UV-2500PC; manufactured and sold byShimadzu Corporation, Japan) under conditions wherein the slit width is5.0 nm and the scan speed is high. Further, the ultraviolet-visiblespectrophotometry for the measurement, in each of Example 1 andComparative Example 1, of the amount of a dye adsorbed is performed withrespect to a wavelength range of from 300 nm to 800 nm using anultraviolet-visible spectrophotometer (trade name: MPC2200; manufacturedand sold by Shimadzu Corporation, Japan) under conditions wherein theslit width is 5.0 nm and the scan speed is high.

(2) Infrared Spectrophotometry (FT-IR) and Liquid Chromatography (LC)

Infrared spectrophotometry is performed as follows. With respect toExample 1, Comparative Example 1, Example 4 and Example 5, themeasurement is performed with respect to a wavenumber range of from 400to 4000 cm⁻¹ in accordance with the KBr method using an infraredspectrophotometer (trade name: SYSTEM 2000 COMPRISIN; manufactured andsold by Perkin Elmer, Inc., U.S.A.) under conditions wherein theresolution is 4 cm⁻¹. With respect to Examples other than Example 1,Comparative Example 1, Example 4 and Example 5, the measurement isperformed with respect to a wavenumber range of from 400 to 4000 cm⁻¹ inaccordance with transmission microscopy using an infraredspectrophotometer (trade name: IRμs; manufactured and sold by SpectraTech, Inc., U.S.A.) under conditions wherein the resolution is 4 cm⁻¹.

Liquid chromatography is performed using a chromatograph (trade name:1100 series; manufactured and sold by Agilent Inc., U.S.A.) and a column(2.1 mm I.D.×150 mm) (trade name: Inertsil ODS-3; manufactured and soldby GL sciences Inc., Japan) under conditions wherein the flow rate is0.2 mL/min, water containing 0.1% by weight of trifluoroacetic acid(TFA) is used as solution A (mobile phase), CH₃CN containing 0.1% byweight of trifluoroacetic acid (TFA) is used as solution B (mobilephase), and the amount of the sample is 3 μL. For the detection, theabsorption of light respectively having wavelengths of 254 nm and 400 nmare used.

(3) Mass Spectrometry (MS)

Two types of mass spectrometry are used. Specifically, electrosprayionization mass spectrometry or matrix-assisted laser desorptionionization/time-of-flight mass spectrometry is employed depending on theproperties of the sample.

Electrospray ionization mass spectrometry (ESI-MS) is performed in thepositive mode with respect to a mass/charge ratio range of from 50 to2000 using a mass spectrometer (trade name: LCQ; manufactured and soldby Thermoquest, U.S.A.). The ionization of the sample is performed byAPCI. As the apparatus for the electrospray ionization, theabove-mentioned apparatus “1100 series” (manufactured and sold byAgilent, U.S.A.) is used.

Matrix-assisted laser desorption ionization/time-of-flight massspectrometry (MALDI-TOF-MS) is performed using a mass spectrometer(trade name: AXIMA CFR plus; manufactured and sold by ShimadzuCorporation, Japan). The ion detector is operated in the linear mode andin the positive ion detection mode, and the number of integration is2000 times. The ionization of the sample is performed by irradiating amatrix having immersed therein the sample, with a nitrogen laser havinga wavelength of 337 nm.

(4) ICP Emission Spectrometry (ICP-AES)

The sample is mixed with nitric acid having a weight 700 times as largeas the weight of the sample. The resultant mixture is decomposed bymicrowaves using a microwave system (trade name: ETHOSPLUS; manufacturedand sold by Milestone, Italy). The resultant decomposed mixture isdiluted with pure water. With respect to the resultant diluted mixture,ICP emission spectrometry is performed using an ICP emissionspectrometer (trade name: JY138; manufactured and sold by JOBIN YVON,France) provided with a cyclone chamber under conditions wherein theflow rate of the plasma gas (PL1) is 13 L/min, the flow rate of thesheath gas (G1) is 0.3 L/min, the nebulizer gas pressure is 3.0 bar, theflow rate of the nebulizer gas is 0.2 L/min and the high-frequency wavepower is 1.4 kW.

(5) Fluorescent X-Ray Analysis

Fluorescent X-ray analysis is performed using a fluorescent X-rayanalyzer (trade name: PW2400; manufactured and sold by PhilipsAnalytical, Netherlands), wherein a rhodium X-ray tube is used.

(6) Cyclic Voltammetry

Cyclic voltammetry is performed using an electrochemical test system(trade name: Solartron 1280Z; manufactured and sold by SOLARTRON, U.K.).As an electrolytic cell, a glass cell (trade name: VC-3; manufacturedand sold by Bioanalytical System (BAS), U.S.A.) is used; as a workingelectrode, glassy carbon or platinum (0.0706 cm²; 3 mmφ) (manufacturedand sold by Bioanalytical System (BAS), U.S.A.) is used; as a counterelectrode, a platinum wire is used; and, as a reference electrode,Ag/Ag⁺ (trade name: RE-5; manufactured and sold by Bioanalytical System(BAS), U.S.A.) is used.

(7) Properties of a Dye-Sensitized Solar Battery

Properties of a dye-sensitized solar battery are measured as follows.

When the dye-sensitized solar battery is a sandwich type solar battery,the measurement is performed as follows. The sandwich type solar batteryis irradiated with a quasi sunray (about 100 mW/cm²) using a solarsimulator (manufactured and sold by WACOM ELECTRIC CO., LTD., Japan),and the short-circuit current (I_(sc)) is measured using an I-V curvetracer (manufactured and sold by EKO INSTRUMENTS Co., Ltd., Japan),wherein the measured area is 1 cm².

On the other hand, when the dye-sensitized solar battery is an immersiontype solar battery, the measurement is performed by the three-electrodetype photoelectrochemical method and/or the two-electrode typephotoelectrochemical method. Specifically, the measurement by thethree-electrode type photoelectrochemical method is performed asfollows. To a 100 ml beaker is fed an electrolytic solution having avolume of about 100 ml. Then, in the beaker containing the electrolyticsolution are immersed an electrode comprising a photoelectric conversionelement as a working electrode, and a platinum wire in the form of acoil as a counter electrode, thereby obtaining an immersion type solarbattery. In the beaker containing the immersion type solar battery isimmersed Ag/Ag⁺ (trade name: RE-5; manufactured and sold byBioanalytical System (BAS), U.S.A.) as a reference electrode. Then,while stirring the electrolytic solution, the measurement by thethree-electrode type photoelectrochemical method is performed using anelectrochemical test system (trade name: Solartron 1280Z; manufacturedand sold by SOLARTRON, U.K.). As the source of light for use in themeasurement of the electric current and voltage generated by the lightray irradiation of the solar battery, a halogen lamp (trade name:AT-100HG; manufactured and sold by Shimadzu Corporation, Japan) isemployed, wherein the irradiation with the halogen lamp is performedusing an apparatus (trade name: PS-150UE-DC; manufactured and sold byShimadzu Corporation, Japan), and, if desired, the amount of theirradiation with the halogen lamp is controlled. The distance betweenthe light source and the reference electrode is about 6 cm.

The measurement by the two-electrode type photoelectrochemical method isperformed in substantially the same manner as in the measurement by thethree-electrode type photochemical method, except that no referenceelectrode is used and a conductor line which, in the case of thethree-electrode type photochemical method, would be connected to areference electrode is connected to a counter electrode.

Example 1 (1) Synthesis of a Composite Dye

(Synthesis of Component Dye Precursor A (Component Dye A) Correspondingto the First Photoabsorption Portion)

0.523 g of ruthenium chloride n-hydrate (reagent; manufactured and soldby Wako Pure Chemical Industries Ltd., Japan) and 50 ml ofdimethylformamide were charged into a three-necked flask, followed bystirring in an atmosphere of nitrogen at room temperature for 15minutes, to thereby obtain a mixture. To the obtained mixture were added50 ml of dimethylformamide and 0.952 g of 4,4′-dicarboxy-2,2′-bipyridine(reagent; manufactured and sold by Tokyo Kasei Kogyo Co., Ltd., Japan).Then, the resultant mixture was refluxed under heating in an atmosphereof nitrogen for 3 hours under conditions wherein light was shielded,thereby obtaining a mixture. The obtained mixture was allowed to cool toroom temperature, followed by a filtration using a filter paper (porediameter: 5 μm) to obtain a filtrate. Subsequently, the obtainedfiltrate was dried by a rotary evaporator, thereby obtaining a solidhaving a dark purple color.

The above-obtained solid was washed with a mixed solvent comprised ofdiisopropyl ether and acetone (diisopropyl ether/acetone volumeratio=4/1). To the washed solid was added 2 N hydrochloric acid,followed by stirring at room temperature for 4 hours under conditionswherein light was shielded. The resultant mixture was subjected to asuction filtration using a filter paper (pore diameter: 1 μm), therebyobtaining component dye A in the form of a solid. Component dye A wasanalyzed by infrared spectrometry, ultraviolet-visible spectrometry andfluorescent X-ray analysis. As a result, it was confirmed that componentdye A was a complex dye comprisingcis-dichloro-bis(2,2′-bipyridyl-4,4′-dicarboxylate)rurthenium(II).

(Synthesis of Component Dye B Corresponding to the SecondPhotoabsorption Portion, which is Bonded to Component Dye A)

1.0 g of potassium tetrachloroplatinate(II) (reagent; manufactured andsold by Wako Pure Chemical Industries Ltd., Japan) was dissolved in 5 mlof purified water. To the resultant solution was added 0.55 ml ofdimethyl sulfoxide, followed by stirring, thereby obtaining a mixture.The obtained mixture was allowed to stand still at room temperature for1 hour, thereby obtaining a crystalline deposit having a light yellowcolor. The obtained crystalline deposit was subjected to a filtrationusing a filter paper (pore diameter: 5 μm), followed by washing withethanol and air-drying.

0.602 g of the dried crystalline deposit was added to a solutionobtained by dissolving 0.301 g of 1,10-phenanthroline-5,6-dione(reagent; manufactured and sold by Sigma-Aldrich Co., U.S.A.) in 60 mlof ethanol. Then, the resultant mixture was refluxed under heating in anatmosphere of air for 3 hours, thereby obtaining a mixture. The obtainedmixture was allowed to cool to room temperature, and subjected to asuction filtration using a filter paper (pore diameter: 5 μm), therebyobtaining powders having a yellowish orange color. The obtained powderswere analyzed by infrared spectrophotometry, liquid chromatography andultraviolet-visible spectrophotometry. As a result, it was confirmedthat the powders were comprised ofdichloro-(1,10-phenanthroline-5,6-dione)platinate(II).

Subsequently, 0.303 g of the powders obtained above were suspended in 80ml of dimethylformamide. To the suspension was added a solution obtainedby dissolving 0.125 g of potassium isothiocyanate (reagent; manufacturedand sold by Wako Pure Chemical Industries Ltd., Japan) in 20 ml ofpurified water, and the resultant mixture was refluxed under heating inan atmosphere of air for 2 hours, thereby obtaining a mixture. Theobtained mixture was allowed to cool to room temperature, and subjectedto a filtration using a filter paper (pore diameter: 5 μm) and thefiltrate was dried using a rotary evaporator, thereby obtaining a solidhaving a dark moss color. The obtained solid was washed with a mixedsolvent comprised of diisopropyl ether and acetone (diisopropylether/acetone volume ratio=4/1), followed by drying, thereby obtainingcomponent dye B in the form of powders. The obtained component dye B wasanalyzed by infrared spectrophotometry, liquid chromatography andfluorescent X-ray analysis. As a result, it was confirmed that componentdye B in the form of powders was a complex dye comprised ofdiisocyanate-(1,10-phenanthroline-5,6-dione)platinate(II).

(Synthesis of Composite Dye Z)

0.100 g of component dye B was dissolved in 30 ml of dimethylformamide,and a solution obtained by dissolving 0.135 g of component dye A in 70ml of methanol was dropwise added thereto at room temperature whilestirring. To the resultant solution was added a solution obtained bydissolving 0.070 g of potassium hexafluorophosphate (reagent;manufactured and sold by Tokyo Kasei Kogyo Co., Ltd., Japan) in 10 ml ofpurified water, followed by stirring at 100° C. for 5 hours. Theresultant mixture was allowed to cool to room temperature, and subjectedto a filtration using a filter paper (pore diameter: 5 μm), therebyobtaining a solid having a dark brown color. The obtained solid waswashed with dimethylformamide and methanol which were, respectively,solvents for component dyes A and B, followed by drying, therebyobtaining composite dye Z having a dark brown color, in which thecomponents were chemically bonded to each other.

Composite dye Z was analyzed by infrared spectrophotometry (IR),ultraviolet-visible spectrophotometry (UV-vis), liquid chromatography(LC), electrospray ionization mass spectrometry (ESI-MS), ICP emissionspectrometry (ICP-AES) and matrix-assisted laser desorptionionization/time-of-flight mass spectrometry (MALDI-TOF-MS). As a resultof the liquid chromatography (LC), it was confirmed that a reactionbetween component dye A and component dye B proceeded; as a result ofthe IR, it was confirmed that 2,2′-bipyridyl-4,4′-dicarboxylate, aderivative of 1,10-phenanthroline-5,6-dione and an isothiocyanate groupwere present; as a result of the ICP-AES, it was confirmed thatruthenium and platinum were present; and, as a result of the ESI-MS andMALDI-TOF-MS in which α-cyano-4-hydroxycinnamic acid was used as amatrix, it was confirmed that a compound having the molecular weight ofthe desired compound was present. The results of the MALDI-TOF-MSanalysis are shown in FIG. 10. The chart in FIG. 10 contains a peakappearing at 1,114 m/z and ascribed to a compound having the desiredmolecular weight, together with a peak ascribed to a compound producedby a decomposition (such as ionization) of the compound having themolecular weight of the desired compound during the measurement, a peakascribed to a compound produced by addition of the above-mentionedmatrix to the compound produced by the decomposition, and a peakappearing near the peak ascribed to the compound having the molecularweight of the desired compound, wherein the peak appearing near the peakascribed to the desired compound was ascribed to a compound in which atleast one of the atoms constituting the desired compound was replaced byan isotope thereof. In FIG. 10, the ordinate shows the percentage ofintensity of a peak, based on the highest peak intensity observed in themeasurement which was performed in the range shown in FIG. 10 (thepercentage is referred to as the “relative peak intensity”). Inaddition, the results of the ultraviolet-visible spectrophotometryperformed with respect to composite dye Z were compared with theabove-obtained results of the ultraviolet-visible spectrophotometryperformed with respect to component dye A and the above-obtained resultsof the ultraviolet-visible spectrophotometry performed with respect tocomponent dye B. As a result, it was confirmed that composite dye Z wasa composite dye comprised of a multinuclear complex in which componentdye A and component dye B are bonded to each other, and that theabsorbances ascribed to component dyes A and B were observed indifferent wavelength ranges in the chart showing the spectra ofcomposite dye Z. From the above, it was confirmed that composite dye Zin the form of powders was a multinuclear complex containing a rutheniumatom and a platinum atom and having a structure in which the chlorideions in component dye A are replaced by a derivative of1,10-phenanthroline-5,6-dione (which is obtained by converting the dioneportion of 1,10-phenanthroline-5,6-dione to oxoanions (O⁻)), so that thederivative is coordinated to component dye A through the oxoanions (O⁻)of the derivative. The representative structure of the multinuclearcomplex is shown in FIG. 11. As shown in FIG. 11, the multinuclearcomplex had a structure represented by formula (1) above. Specifically,the multinuclear complex had a structure comprising: bipyridyl rings(each corresponding to L₀ in formula (1)) each having carboxyl groups asa binding functional group (wherein a part of the carboxyl groups may beconverted to a potassium-containing group represented by the formula:COOK); a ruthenium atom (corresponding to M₀ in formula (1)) coordinatedto the bipyridyl rings; a 1,10-phenanthroline-5,6-dione derivative(corresponding to BL in formula (1)) as a bridging ligand coordinated tothe ruthenium atom, wherein the dione portion as a non-heterocyclicsegment is converted to oxoanion (O⁻); a platinum atom (corresponding toM in formula (1)) coordinated to the heterocyclic segment of thederivative; and isothiocyanate groups (anionic ligands) (eachcorresponding to X in formula (1)) coordinated to the platinum atom.Therefore, it was confirmed that component dye A and component dye B ascomplex dyes had been chemically bonded to each other. Thus, it wasconfirmed that the dye produced in this Example 1 was a composite dyewherein a plurality of component dyes are chemically bonded to eachother.

0.0035 g of composite dye Z (i.e., a multinuclear complex comprised ofcomponent dye A and component dye B) was dissolved in 25 ml ofdimethylformamide, followed by a filtration using a filter paper (porediameter: 1 μm). The resultant filtrate was subjected to cyclicvoltammetry under conditions wherein 0.975 g of tetra-t-butylammoniumhexafluorophosphate was used as a supporting electrolyte, anelectrolytic cell was purged with nitrogen gas, the voltage scanningrate was 20 mV/sec and a glassy carbon electrode was used as a workingelectrode. As a result, it was found that an oxidation wave derived fromruthenium was observed at 0.75 V (relative to the electric potential ofthe reference electrode) and that an oxidation wave derived fromplatinum was observed at 1.0 V (relative to the electric potential ofthe reference electrode). Thus, it was confirmed that a portion derivedfrom component dye A corresponding to the first photoabsorption portionhad a high excitation level. The determination of the substances fromwhich the oxidation waves were derived was performed, taking intoconsideration the results of the cyclic voltammetry performed withrespect to component dyes A and B. From the above, it was confirmed thatcomposite dye Z had different excitation levels.

(2) Preparation of a Photoelectric Conversion Element

(Preparation of N-Type Semiconductor Dispersion)

6 g of small particles of crystalline titanium oxide (P-25; manufacturedand sold by NIPPON AEROSIL CO., LTD., Japan), 120 g of water and 1.49 gof nitric acid were mixed together, followed by heating at 80° C. forabout 8 hours. The resultant mixture was allowed to cool to roomtemperature, and subjected to a distillation using an evaporator toremove water from the mixture, thereby obtaining powders. The obtainedpowders were pulverized using a mortar, thereby obtaining crystallinetitanium oxide in the form of small particles. 1 g of the obtainedcrystalline titanium oxide and 3.68 g of water were mixed together usingan ultrasonic homogenizer for about 10 minutes. To the resultantdispersion were gradually added 1 g of a 1.7% by weight aqueous titaniumperoxide solution (PTA; manufactured and sold by TANAKA TENSHA CO.,LTD., Japan) as a sintering additive and 0.06 g of Triton X-100(surfactant; manufactured and sold by Sigma-Aldrich Co., U.S.A.) whilestirring, thereby preparing an n-type semiconductor dispersion.

(Preparation of a Photoelectric Conversion Element)

Using a wire bar (length of wired portion: 300 m/m; core diameter: 12.5m/m; wire diameter: 0.20 m/m), the above-obtained dispersion was appliedto the electroconductive side-surface of a transparent electroconductiveglass (manufactured and sold by Nippon Sheet Glass Co., Ltd., Japan)wherein a layer of fluorine-doped tin oxide (FTO, resistance of sheet:about 8Ω/□) was provided on a glass substrate, followed by air-drying atroom temperature for 1 hour, thereby forming an n-type semiconductormembrane on the transparent electroconductive glass. The transparentelectroconductive glass having carried thereon the semiconductormembrane obtained above was sintered at 500° C. for about 30 minutesusing an electric furnace. The resultant, sintered membrane had athickness of about 1.7 μm.

Subsequently, into dimethyl sulfoxide was charged composite dye Z in anamount such that when composite dye Z was completely dissolved in thedimethyl sulfoxide, the concentration of composite dye Z in the dimethylsulfoxide became 3.7×10⁻⁴ mol/l, thereby obtaining a mixture in which apart of composite dye Z was dissolved in dimethyl sulfoxide. Theobtained mixture was refluxed under heating for about 45 minutestogether with the semiconductor membrane above, thereby obtaining aphotoelectric conversion element in which composite dye Z was carried(adsorbed) on the semiconductor membrane. The obtained photoelectricconversion element was in the form of an electrode (i.e., photoelectricconversion electrode) comprising the photoelectric conversion elementand the transparent electroconductive substrate. Then, the electrode wasroughly washed with acetonitrile. The amount of composite dye Z carriedon the semiconductor membrane was 0.90×10⁻⁸ mol/cm². Even when theelectrode was washed with dimethyl sulfoxide as a solvent for compositedye Z, composite dye Z carried on the semiconductor membrane was notdetached from the semiconductor. The electrode was analyzed by IR. As aresult, it was confirmed that composite dye Z was chemically bonded tothe n-type semiconductor.

(3) Preparation of a Dye-Sensitized Solar Battery and Evaluationsthereof

A platinum membrane having a thickness of about 0.1 μm was formed on aslide glass by a sputtering method, thereby obtaining a platinumelectrode comprising a slide glass and a platinum membrane. The obtainedplatinum electrode was used as a counter electrode. On the other hand,an electrolytic solution was prepared as follows. Tomethoxypropionitrile as a solvent were added iodine (reagent;manufactured and sold by Wako Pure Chemical Industries, Ltd., Japan),lithium iodide (reagent; manufactured and sold by Wako Pure ChemicalIndustries, Ltd., Japan), dimethylpropyl imidazolium iodide (DMPII;manufactured and sold by SOLARONIX, Switzerland) and tert-butyl pyridine(reagent; manufactured and sold by Tokyo Kasei Kogyo Co., Ltd., Japan)(wherein iodine, lithium iodide and dimethylpropyl imidazolium iodidewere used as electrolytes and tert-butyl pyridine was used as anadditive), thereby preparing an electrolytic solution. Theconcentrations of iodine, lithium iodide, dimethylpropyl imidazoliumiodide and tert-butyl pyridine in the electrolytic solution were 0.05mol/l, 0.1 mol/l, 0.6 mol/l and 0.5 mol/l, respectively.

The thus prepared electrolytic solution was dropwise applied to thephotoelectric conversion electrode, and the photoelectric conversionelectrode, the counter electrode and the electrolytic solution were soarranged that the electrolytic solution was sandwiched between theplatinum surface of the counter electrode and the photoelectricconversion element, thereby preparing a dye-sensitized solar battery (asandwich type).

(Evaluation of Photoelectric Conversion Properties)

The prepared dye-sensitized solar battery was connected to a measuringinstrument, and photoelectric conversion properties of the solar batterywere measured while irradiating the solar battery with a quasi sunrayhaving a light intensity of about 100 mW/cm². As a result, it wasconfirmed that the electric current per molar amount of the dye adsorbedwas 2.2×10⁸ mA/mol. When a part of the sunray was shut out by using a UVcut filter (L41 Super PRO WIDE; manufactured and sold by Kenko Co.,Ltd., Japan), the electric current per molar amount of the dye adsorbedwas 1.3×10⁸ mA/mol. These data were compared with the data obtained inComparative Example 1 mentioned below. As a result, it was confirmedthat about 30% of the electric current was obtained from a portionderived from component dye B and, hence, the transfer of electronsoccurred between the two photoabsorption portions forming composite dyeZ (multinuclear complex).

Example 2 (1) Synthesis of a Component Dye

(Synthesis of a Bridging Ligand)

1 g of 1,10-phenanthroline-5,6-dione (reagent; manufactured and sold bySigma-Aldrich Co., U.S.A.) and 0.691 g of dithioxamide (reagent;manufactured and sold by Tokyo Kasei Kogyo Co., Ltd., Japan) were addedto 40 ml of ethanol, and the resultant mixture was refluxed underheating in an atmosphere of air for 13 hours. The resultant reactionmixture was allowed to cool to room temperature, and subjected to asuction filtration using a filter paper (pore diameter: 1 μm), therebyobtaining powders. For removing substances remaining unreacted in theobtained powders, the powders were washed with 100 ml of chloroform atroom temperature while stirring, followed by a suction filtration usinga filter paper (pore diameter: 1 μm), thereby obtaining 0.759 g of5,6-dihydroxy-1,10-phenanthroline in the form of powders.

(Synthesis of Multinuclear Complex Precursor C (Bonding of a BridgingLigand to the First Photoabsorption Portion))

0.111 g of(cis-dichloro-bis(2,2′-bipyridyl-4,4′-dicarboxylate)rurthenium(II)(manufactured and sold by Kojima Chemicals Co., LTD., Japan) (which wasused as a precursor for the first photoabsorption portion), 0.04 g ofthe above-synthesized 5,6-dihydroxy-1,10-phenanthroline and 0.055 g ofpotassium hydroxide (reagent; manufactured and sold by Wako PureChemical Industries, Ltd., Japan) were added to a mixed solventcomprised of 14 ml of dimethylformamide and 7 ml of purified water.Then, the resultant mixture was refluxed under heating in an atmosphereof nitrogen for 3 hours. The resultant mixture was allowed to cool toroom temperature, and dried using a rotary evaporator, thereby obtaininga solid having a brown color.

The obtained solid was dissolved in 8 ml of purified water, followed byaddition of 3 ml of 0.1 N hydrochloric acid, thereby obtaining a liquidhaving deposited therein powders having a dark brown color. The liquidcontaining the powders was subjected to a centrifugation using acentrifugal separator at 12,000 r/m for 5 minutes, thereby recoveringpowders. The recovered powders were washed with 20 ml of ethanol,followed by vacuum drying, thereby obtaining purified powders. Theobtained purified powders were analyzed by infrared spectrophotometry,ultraviolet-visible spectrophotometry, and matrix-assisted laserdesorption ionization/time-of-flight mass spectrometry (MALDI-TOF-MS)using α-cyano-4-hydroxycynnamic acid as a matrix. As a result, it wasconfirmed that the powders were a multinuclear complex precursorcomprised of(bis(2,2′-bipyridyl-4,4′-dicarboxylate)-(1,10-phenanthroline-5,6-diolate))rurthenium(II),wherein a part of the carboxylic acid portions thereof might beconverted to a potassium-containing group represented by the formula:COOK (hereinafter, this multinuclear complex precursor is referred to as“multinuclear complex precursor C”). The results of the MALDI-TOF-MSanalysis are shown in FIG. 12. The chart in FIG. 12 contains a peakappearing at 801 m/z and ascribed to a compound having the desiredmolecular weight, together with a peak ascribed to a compound producedby a decomposition (such as ionization) of the compound having themolecular weight of the desired compound during the measurement, a peakascribed to a compound produced by addition of the above-mentionedmatrix to the compound produced by the decomposition, and a peakappearing near the peak ascribed to the compound having the molecularweight of the desired compound, wherein the peak appearing near the peakascribed to the desired compound was ascribed to a compound in which atleast one of the atoms constituting the desired compound was replaced byan isotope thereof. In FIG. 12, the ordinate shows the percentage ofintensity of a peak, based on the highest peak intensity observed in themeasurement which was performed in the range shown in FIG. 12 (thepercentage is referred to as the “relative peak intensity”). Therepresentative structure of multinuclear complex precursor C is shown inFIG. 13. As shown in FIG. 13, multinuclear complex precursor C had astructure comprising: bipyridyl rings (corresponding to L₀ in formula(1) above) each having carboxyl groups as a binding functional group(wherein a part of the carboxyl groups may be converted to apotassium-containing group represented by the formula: COOK); aruthenium atom (corresponding to M₀ in formula (1)) coordinated to thebipyridyl rings; and a bridging ligand (corresponding to BL in formula(1)) comprised of a heterocyclic segment and a non-heterocyclic segment,wherein the bridging ligand was coordinated to the ruthenium atomthrough the non-heterocyclic segment.

(Synthesis of Composite Dye Y)

0.05 g of multinuclear complex precursor C was added to 200 ml of amixed solvent comprised of 100 ml of ethanol (which had been subjectedto nitrogen bubbling) and 100 ml of purified water, followed by stirringunder heating, thereby obtaining a solution. Separately, 0.029 g of(cis-dichloro-bis(2,2′-bipyridyl))rurthenium(II) dihydrate (reagent;manufactured and sold by Sigma-Aldrich Co., U.S.A.) as a precursor forthe second photoabsorption portion was added to 200 ml of a mixedsolvent comprised of 100 ml of ethanol (which had been subjected tonitrogen bubbling) and 100 ml of purified water, followed by stirring atroom temperature, thereby obtaining another solution. The thus obtainedtwo types of solutions were mixed together. The resultant mixture wasrefluxed under heating in an atmosphere of nitrogen for 1 hour. Theresultant mixture was allowed to cool to room temperature, and subjectedto a filtration using a filter paper (pore diameter: 5 μm). Theresultant filtrate was dried using a rotary evaporator, therebyobtaining powders having a brown color.

The brown powders were analyzed by infrared spectrophotometry (IR),ultraviolet-visible spectrophotometry (UV-vis) and matrix-assisted laserdesorption ionization/time-of-flight mass spectrometry (MALDI-TOF-MS).As a result of the IR, it was confirmed that2,2′-bipyridyl-4,4′-dicarboxylic acid and a potassium salt thereof,1,10-phenanthroline-5,6-diolate, and 2,2′-bipyridyl were present, andthat the bonding between ruthenium and the diolate portion of1,10-phenanthroline-5,6-diolate was present. In addition, the results ofthe ultraviolet-visible spectrophotometry performed with respect to thebrown powders were compared with the results of the ultraviolet-visiblespectrophotometry which had been separately performed with respect tomultinuclear complex precursor C and the results of theultraviolet-visible spectrophotometry which had been separatelyperformed with respect to the(cis-dichloro-bis(2,2′-bipyridyl))rurthenium(II) dihydrate. As a result,it was confirmed that the brown powders were a multinuclear complex dye(i.e., a composite dye) obtained by bonding multinuclear complexprecursor C and (cis-dichloro-bis(2,2′-bipyridyl))rurthenium(II)dehydrate, and that the absorbances ascribed to multinuclear complexprecursor C and (cis-dichloro-bis(2,2′-bipyridyl))rurthenium(II)dihydrate were observed in different wavelength ranges in the chartshowing the spectra of the brown powders. Further, by MALDI-TOF-MSanalysis using 2,5-dihydroxy benzoic acid as a matrix, it was confirmedthat a compound having the molecular weight of the desired compound waspresent. The results of the MALDI-TOF-MS analysis are shown in FIG. 14.The chart in FIG. 14 contains a peak appearing at 1,214 m/z and ascribedto a compound having the desired molecular weight, together with a peakascribed to a compound produced by a decomposition (such as ionization)of the compound having the molecular weight of the desired compoundduring the measurement, a peak ascribed to a compound produced byaddition of the above-mentioned matrix to the compound produced by thedecomposition, and a peak appearing near the peak ascribed to thecompound having the molecular weight of the desired compound, whereinthe peak appearing near the peak ascribed to the desired compound wasascribed to a compound in which at least one of the atoms constitutingthe desired compound was replaced by an isotope thereof. In FIG. 14, theordinate shows the percentage of intensity of a peak, based on thehighest peak intensity observed in the measurement which was performedin the range shown in FIG. 14 (the percentage is referred to as the“relative peak intensity”). From the above, it was confirmed that thepowders were a chloride salt of a multinuclear complex containing tworuthenium atoms (i.e.,(bis(2,2′-bipyridyl-4,4′-dicarboxylate)-(1,10-phenanthroline-5,6-diolate))rurthenium(II)-(bis-(2,2′-bipyridyl))rurthenium(II)).Specifically, the multinuclear complex had a structure in which thechloride ions of (cis-dichloro-bis(2,2′-bipyridyl))rurthenium(II)dehydrate are replaced by 1,10-phenanthroline-5,6-diolate (which isderived from multinuclear complex precursor C), so that1,10-phenanthroline-5,6-diolate is coordinated to(cis-dichloro-bis(2,2′-bipyridyl))rurthenium(II) dihydrate through thephenanthroline portion of 1,10-phenanthroline-5,6-diolate (wherein apart of the carboxylic acid portions thereof may be converted to apotassium-containing group represented by formula: COOK) (hereinafter,this multinuclear complex is referred to as “multinuclear complex(composite dye) Y”). The representative structure of the multinuclearcomplex is shown in FIG. 15. As shown in FIG. 15, composite dye Y had astructure represented by the formula (1) above. Specifically, compositedye Y had a structure comprising: bipyridyl rings (each corresponding toL₀ in formula (1)) having carboxyl groups as a binding functional group(wherein a part of the carboxyl groups may be converted to apotassium-containing group represented by the formula: COOK); a firstruthenium atom (corresponding to M₀ in formula (1)) coordinated to thebipyridyl groups; 1,10-phenanthroline-5,6-diolate (corresponding to BLin formula (1)) as a bridging ligand comprised of a heterocyclic segmentand a non-heterocyclic segment (the diolate portion of1,10-phenanthroline-5,6-diolate), wherein the bridging ligand wascoordinated to the first ruthenium atom through the diolate portion asthe non-heterocyclic segment thereof; a second ruthenium atom(corresponding to M in formula (1)) coordinated to the heterocyclicsegment of the bridging ligand; and bipyridyl rings (each correspondingto L in formula (1)) as a heterocyclic ring coordinated to the secondruthenium atom. Thus, it was confirmed that the brown powders were acomposite dye comprising a plurality of component dyes which werechemically bonded to each other.

0.006 g of multinuclear complex Y was dissolved in 10 ml of acetonitrileto thereby obtain a solution. The obtained solution was subjected tocyclic voltammetry under conditions wherein 0.342 g oftetra-n-butylammonium perchlorate was used as a supporting electrolyte,an electrolytic cell was purged with nitrogen gas, the voltage scanningrate was 20 mV/sec and a platinum electrode was used as a workingelectrode. As a result, it was found that an oxidation wave derived fromruthenium contained in multinuclear complex precursor C corresponding tothe first photoabsorption portion was observed at 0.59 V (relative tothe electric potential of the reference electrode) and that an oxidationwave derived from the portion (of(cis-dichloro-bis(2,2′-bipyridyl))rurthenium(II) dihydrate correspondingto the second photoabsorption portion) having chloride ions detachedtherefrom and bonded to the first photoabsorption portion was observedat 1.1 V (relative to the electric potential of the referenceelectrode). Thus, it was confirmed that the photoabsorption portionpositioned on the side of the n-type semiconductor had a higherexcitation level. The determination of the substances from which theoxidation waves were derived was performed, taking into considerationthe results of the cyclic voltammetry which was performed with respectto (cis-dichloro-bis(2,2′-bipyridyl-4,4′-dicarboxylate)rurthenium(II)(wherein the cyclic voltammetry was performed in substantially the samemanner as mentioned above, except that dimethylformamide was usedinstead of acetonitrile) and the results of the cyclic voltammetry whichwas performed with respect to(1,10-phenanthroline-5,6-dione)-(bis(2,2′-bipyridyl))rurthenium(II)(wherein the cyclic voltammetry was performed in the same manner asmentioned above (i.e., in a manner such that acetonitrile was used), orin substantially the same manner as mentioned above, except thatdimethylformamide was used instead of acetonitrile). From the above, itwas confirmed that composite dye Y had different excitation levels.

(2) Preparation of a Photoelectric Conversion Element, and Preparationand Evaluation of a Dye-Sensitized Solar Battery

An n-type semiconductor in the form of a membrane was formed on atransparent electroconductive glass in substantially the same manner asin Example 1, except that the wire diameter of the wire bar was changedto 1.0 m/m (therefore, the preparation of an n-type semiconductordispersion, the application of the dispersion to the transparentelectroconductive glass, the drying of the resultant, and the sinteringwere performed in the same manner as in Example 1). The thickness of thesintered membrane was about 8 μm. Subsequently, a dye-sensitized solarbattery (a sandwich type) was prepared in substantially the same manneras in Example 1, except that dimethyl sulfoxide was changed to ethanoland that composite dye Z was changed to composite dye Y, wherein theamount of composite dye Y was such that when composite dye Y wascompletely dissolved in ethanol, the concentration of composite dye Y inethanol became 3.0×10⁻⁴ mol/l. Even when the electrode was washed withethanol as a solvent for composite dye Y, composite dye Y carried on thesemiconductor membrane was not detached from the semiconductor membraneand, hence, it was confirmed that composite dye Y was secured to then-type semiconductor. The photoelectric conversion properties of thedye-sensitized solar battery were evaluated in substantially the samemanner as in Example 1. As a result, it was found that the electriccurrent was 0.48 mA/cm². The data obtained by the evaluation werecompared with the data obtained in Comparative Example 2 mentionedbelow. As a result, it was confirmed that a part of the electric currentwas obtained from a portion derived from(cis-dichloro-bis(2,2′-bipyridyl))rurthenium(II) dehydrate, whichcorresponded to the second photoabsorption portion and was positioned ona side remote from the n-type semiconductor and, hence, the transfer ofelectrons occurred between the two photoabsorption portions of themultinuclear complex.

Example 3 (1) Production of a Photoelectric Conversion Element

An n-type semiconductor dispersion was prepared in substantially thesame manner as in Example 1. The prepared dispersion was applied to theelectroconductive side-surface of a transparent electroconductive glass(manufactured and sold by Nippon Sheet Glass Co., Ltd., Japan) in whichan FTO layer (2.5 cm×5 cm) was provided on a glass substrate, whereinthe application of the dispersion was performed in substantially thesame manner as in Example 1, except that the wire diameter of the wirebar was changed to 1.0 m/m. Subsequently, the resultant was subjected todrying and sintering in substantially the same manner as in Example 1,followed by adsorption of multinuclear complex Y in substantially thesame manner as in Example 2, to thereby obtain a photoelectricconversion electrode (photoelectric conversion element).

(2) Preparation of an Electrolytic Solution and PhotoelectrochemicalMeasurement

Tetra-n-butylammonium chloride (reagent; manufactured and sold by TokyoKasei Kogyo Co., Ltd., Japan) as an electrolyte was dissolved inacetonitrile in a concentration of 0.1 mol/l, thereby obtaining anelectrolytic solution. The above-obtained photoelectric conversionelectrode as a working electrode and a platinum wire as a counterelectrode were immersed in the electrolytic solution, thereby obtainingan immersion type solar battery. With respect to the obtained immersiontype solar battery, the photoelectric conversion properties thereof weremeasured by the two-electrode type photoelectrochemical method and thethree-electrode type photoelectrochemical method under conditionswherein the voltage applied to the source of light was 12 V and thevoltage scanning rate was 20 mV/sec. As a result, generation of electriccurrent by the light ray irradiation was observed. It was found that thevoltage generated by the light ray irradiation was 1.2 V and that thecounter electrode exhibited a potential of +0.1 V, relative to thepotential of the reference electrode.

Example 4 (1) Synthesis of an Electrolyte

3.59 g of cobalt(II) chloride hexahydrate and 5.01 g oftetraethylammonium chloride (both manufactured and sold by Wako PureChemical Industries, Ltd., Japan) were individually dissolved in 15 mlof ethanol which had been dehydrated using a molecular sieve, therebyobtaining two ethanol solutions. The obtained two ethanol solutions weremixed together, and the resultant mixture was refluxed under heating forabout 10 minutes to effect a reaction. The resultant reaction mixturewas allowed to cool to room temperature to thereby deposit powdershaving a blue color. The reaction mixture was subjected to filtration torecover the blue powders, followed by drying, thereby obtaining driedpowders. The dried powders obtained were analyzed by IR and XRF. As aresult, it was confirmed that the dried powders weretetrachlorocobalt(II)-bis(tetraethylammonium) ((Et₄N)₂[COCl₄]).

(2) Production of a Photoelectric Conversion Electrode, Preparation ofan Electrolytic Solution, and Photoelectrochemical Measurement

The production of a photoelectric conversion electrode, the preparationof an electrolytic solution, and the photoelectrochemcial measurementwere performed in substantially the same manner as in Example 3, exceptthat the electrolytic solution was changed to a 0.05 mol/l acetonitrilesolution of the above-obtainedtetrachlorocobalt(II)-bis(tetraethylammonium). As a result, generationof electric current by the light ray irradiation was observed. It wasfound that the voltage generated by the light ray irradiation was 0.8 Vand that the counter electrode exhibited a potential of 0 V, relative tothe potential of the reference electrode.

Example 5

The production of a photoelectric conversion electrode was performed insubstantially the same manner as in Example 4, except that, instead ofcomposite dye Y, composite dye Z produced in Example 1 was added to andpartially dissolved in dimethyl sulfoxide (wherein the amount ofcomposite dye Z was such that when composite dye Z was completelydissolved in the dimethyl sulfoxide, the concentration of composite dyeZ in the resultant solution became 3.0×10⁻⁴ mol/l) and the resultantmixture was refluxed under heating for about 1 hour together with thesemiconductor membrane, thereby obtaining a photoelectric conversionelectrode. On the other hand, the preparation of an electrolyticsolution and the photoelectrochemical measurement were performed insubstantially the same manner as in Example 4, except that, in thepreparation of the electrolytic solution, nitrosonium borontetrafluoride (reagent; manufactured and sold by Sigma-AldrichCorporation, U.S.A.) was further used in an amount such that theconcentration of nitrosonium boron tetrafluoride in the electrolyticsolution was 0.5 mmol/l. As a result, generation of electric current bythe light ray irradiation was observed. It was found that the voltagegenerated by the light ray irradiation was 0.8 V and that the counterelectrode exhibited a potential of +0.4 V, relative to the potential ofthe reference electrode.

Example 6 (1) Synthesis of an Electrolyte

0.6 g of potassium hexachlororhenate(IV) (reagent; manufactured and soldby Wako Pure Chemical Industries Ltd., Japan) was dissolved in 50 ml of0.1 N hydrochloric acid. To the resultant solution was added 6.54 g of a10% by weight aqueous solution of tetra-n-butylammonium hydroxide(reagent; manufactured and sold by Tokyo Kasei Kogyo Co., Ltd., Japan)while stirring, thereby effecting a reaction to form a precipitate. Theprecipitate was recovered by filtration, followed by washing with apurified water and drying, thereby obtaining a dried precipitate. Thedried precipitate obtained was analyzed by IR and XRF. As a result, itwas confirmed that the dried precipitate was tetra-n-butylammoniumhexachlororhenate(IV) ((n-BU₄N)₂[ReCl₆]).

(2) Production of a Photoelectric Conversion Electrode, Preparation ofan Electrolytic Solution, and Photoelectrochemical Measurement

An electrolytic solution was prepared in substantially the same manneras in Example 3, except that, as the electrolytic solution, there wasused a 0.01 mol/l acetonitrile solution of the above-obtainedtetra-n-butylammonium hexachlororhenate(IV), wherein the solutionfurther contained, as an additive, nitrosonium boron tetrafluoride(reagent; manufactured and sold by Sigma-Aldrich Corporation, U.S.A.) ina concentration of 0.2 mmol/l. On the other hand, a photoelectricconversion electrode was produced in substantially the same manner as inExample 3, except that a titanium oxide layer having a thickness ofabout 0.2 μm was formed on the FTO layer of the transparentelectroconductive glass used in Example 3 by a sputtering method using atitanium oxide target and the n-type semiconductor was applied to thetitanium oxide layer. Further, the photoelectrochemical measurement wasperformed under conditions wherein a UV filter (trade name: L42;manufactured and sold by Kenko Co., Ltd., Japan) was provided betweenthe source of light and the working electrode (i.e., photoelectricconversion electrode). As a result, generation of electric current bythe light ray irradiation was observed. It was found that the voltagegenerated by the light ray irradiation was 1.1 V and that the counterelectrode exhibited a potential of +0.7 V, relative to the potential ofthe reference electrode.

Example 7

A photoelectric conversion electrode (hereinafter referred to as“photoelectric conversion electrode S”) was produced in substantiallythe same manner as in Example 3. Another photoelectric conversionelectrode (hereinafter referred to as “photoelectric conversionelectrode T”) was produced in substantially the same manner as inExample 3, except that, instead of composite dye Y,cis-bis(isothiocyanato)bis(2,2′-bipyridyl-4,4′-dicarboxylato)-ruthenium(II)bis-tetrabutylammonium (trade name: Ruthehium 535-bis TBA; manufacturedand sold by Solaronix SA, Switzerland) was used as a composite dye.

Subsequently, an electrolytic solution was prepared by dissolvinglithium iodide (reagent; manufactured and sold by Wako Pure ChemicalIndustries Ltd., Japan) as an electrolyte in acetonitrile in aconcentration of 0.1 mol/l (two batches of the solution were prepared).The above-obtained photoelectric conversion electrodes S and T wererespectively immersed in the two batches of the solution prepared above,thereby obtaining two types of immersion type solar batteries. Withrespect to each of the immersion type solar batteries, the electriccurrents obtained by the solar battery were measured by thethree-electrode type photoelectrochemical method under conditionswherein an electric potential of −0.2 V was applied to the workingelectrode (i.e., photoelectric conversion electrode) and the light rayirradiation of the solar battery was performed while increasing thevoltage applied to the source of light from 4 V to 11 V so as to varythe intensity of light, wherein the applied voltage was increased atfirst by 0.5 V from 4 V to 8 V, then by 1 V from 8 V to 11 V. Then, theratios of the electric current obtained by the solar battery usingphotoelectric conversion electrode S to the electric current obtained bythe solar battery using photoelectric conversion electrode T (whereinthe electric currents were generated by the light ray irradiation of thesolar batteries when the voltages applied to the source of light werethe above-mentioned values) were calculated. As a result, the followingratios were obtained, wherein the ratios are expressed in terms ofrelative values such that the relative value was 1 when the voltageapplied to the source of light was 11 V (i.e., when the intensity oflight was high) and arranged in the order of voltage increase from 4 V(wherein the intensity of light is low): 0.64, 0.71, 0.75, 0.80, 0.84,0.86, 0.88, 0.91, 0.93, 0.95, 0.97 and 1. These results are shown inFIG. 16. Specifically, in FIG. 16, the abscissa shows the voltagesapplied to the source of light, and the ordinate shows theabove-mentioned relative values such that the relative value was 1 whenthe voltage applied to the source of light was 11 V (this relative valueis hereinafter referred to as “relative value of the electric currentratio”). It was observed that the electric current was small when theirradiated light had a low intensity. Such a phenomenon ischaracteristic to the two-photon absorption process, which is apparentfrom the comparison between the results of this Example 7 and thebelow-mentioned Reference Example 1.

Example 8

The production of photoelectric conversion electrodes, and thepreparation of an electrolytic solution were performed in substantiallythe same manner as in Example 7, except that, in the production ofphotoelectric conversion electrode T, instead of composite dye Y,composite dye Z produced in Example 1 was added to and partiallydissolved in dimethyl sulfoxide (wherein the amount of composite dye Zwas such that when composite dye Z was completely dissolved in thedimethyl sulfoxide, the concentration of composite dye Z in theresultant solution became 3.0×10⁻⁴ mol/l) and the resultant mixture wasrefluxed under heating for about 1 hour together with the semiconductormembrane, thereby obtaining a photoelectric conversion electrode (thisphotoelectric conversion electrode is hereinafter referred to as“photoelectric conversion electrode U”). The ratios of the electriccurrent obtained by the immersion type solar battery using photoelectricconversion electrode U to the electric current obtained by the immersiontype solar battery using photoelectric conversion electrode T weremeasured by the three-electrode type photoelectrochemical method insubstantially the same manner as in Example 7. As a result, thefollowing ratios were obtained, wherein the ratios are expressed interms of relative values such that the relative value was 1 when thevoltage applied to the source of light was 11 V (i.e., when theintensity of light was high) and arranged in the order of voltageincrease from 4 V (wherein the intensity of light is low): 0.64, 0.73,0.78, 0.82, 0.86, 0.89, 0.91, 0.94, 0.95, 0.97, 1.00 and 1. Theseresults are shown in FIG. 17. Specifically, in FIG. 17, the abscissashows the voltages applied to the source of light, and the ordinateshows the above-mentioned relative values such that the relative valuewas 1 when the voltage applied to the source of light was 11 V (thisrelative value is hereinafter referred to as “relative value of theelectric current ratio”).

Comparative Example 1

A dye-sensitized solar battery (a sandwich type solar battery) wasproduced in substantially the same manner as in Example 1, except that,in the production of a photoelectric conversion electrode, instead ofcomposite dye Z, an ethanol solution of a photosensitizer comprised ofcis-bis(isothiocyanato)bis(2,2′-bipyridyl-4,4′-dicarboxylato)-ruthenium(II)bis-tetrabutylammonium (trade name: Ruthehium 535-bis TBA; manufacturedand sold by Solaronix SA, Switzerland) (wherein the concentration of thesensitizer was 3×10⁻⁴ mol/l) was used as a dye. The amount of theabove-mentioned photosensitizer (i.e., dye) adsorbed was 3.2×10⁻⁸mol/cm².

With respect to the above-mentioned solar battery, the photoelectricconversion properties thereof were measured in substantially the samemanner as in Example 1. As a result, it was found that the amount ofelectric current per molar amount of the dye adsorbed was 1.2×10⁸mA/mol. It was also found that, when a UV filter was provided betweenthe source of light and the working electrode (i.e., photoelectricconversion electrode), the electric current per molar amount of the dyeadsorbed was 1.0×10⁸ mA/mol.

Comparative Example 2

A dye-sensitized solar battery (a sandwich type solar battery) wasproduced in substantially the same manner as in Example 2, except that,in the production of a photoelectric conversion electrode, instead ofcomposite dye Y, multinuclear complex precursor C was used as a dye.With respect to the above-mentioned solar battery, the photoelectricconversion properties thereof were measured in substantially the samemanner as in Example 2. As a result, it was found that the short-circuitcurrent (I_(sc)) was 0.12 mA/cm². Further, the amount of the dyeadsorbed was measured by a method in which the dye (i.e., multinuclearcomplex precursor C) is separated from the semiconductor using a 0.1mol/l aqueous solution of KOH. As a result, it was found that the amountof the dye adsorbed was 75 mol % per unit area, based on the molaramount of composite dye Y adsorbed in Example 2.

Comparative Example 3

The production of a photoelectric conversion electrode, the preparationof an electrolytic solution, and the photoelectrochemical measurementwere performed in substantially the same manner as in Example 3, exceptthat, in the production of the photoelectric conversion electrode,instead of composite dye Y, a complex dye comprised ofcis-bis(isothiocyanato)bis(2,2′-bipyridyl-4,4′-dicarboxylato)-ruthenium(II)bis-tetrabutylammonium (trade name: Ruthehium 535-bis TBA; manufacturedand sold by Solaronix SA, Switzerland) was used as a dye, and that theelectrolytic solution was prepared using lithium iodide as anelectrolyte (reagent; manufactured and sold by Wako Pure ChemicalIndustries, Ltd., Japan). As a result, generation of electric current bythe light ray irradiation was observed. It was found that the voltagegenerated by the light ray irradiation was 0.7 V and that the counterelectrode exhibited a potential of −0.3 V, relative to the potential ofthe reference electrode.

Comparative Example 4

The production of a photoelectric conversion electrode, the preparationof an electrolytic solution, and the photoelectrochemcial measurementwere performed in substantially the same manner as in Example 6, exceptthat, in the production of the photoelectric conversion electrode,instead of composite dye Y, a complex dye comprised ofcis-bis(isothiocyanato)bis(2,2′-bipyridyl-4,4′-dicarboxylato)-ruthenium(II)bis-tetrabutylammonium (trade name: Ruthehium 535-bis TBA; manufacturedand sold by Solaronix SA, Switzerland) was used as a dye. As a result,it was found that generation of electric current by the light rayirradiation was not observed.

Reference Example 1

Two of photoelectric conversion electrode T produced in Example 7 wereprovided (these electrodes are hereinafter referred to as “photoelectricconversion electrode T1” and “photoelectric conversion electrode T2”,respectively). The ratios of the electric current obtained by theimmersion type solar battery using photoelectric conversion electrode T1to the electric current obtained by the immersion type solar batteryusing photoelectric conversion electrode T2 were measured by thethree-electrode type photoelectrochemical method in substantially thesame manner as in Example 7. As a result, the following ratios wereobtained, wherein the ratios are expressed in terms of relative valuessuch that the relative value was 1 when the voltage applied to thesource of light was 11 V (i.e., when the intensity of light was high)and arranged in the order of voltage increase from 4 V (wherein theintensity of light is low): 1.00, 1.01, 0.99, 0.99, 0.98, 0.98, 0.99,1.01, 0.99, 1.01, 1.02 and 1. These results are shown in FIG. 18.Specifically, in FIG. 18, the abscissa shows the voltages applied to thesource of light, and the ordinate shows the above-mentioned relativevalues such that the relative value was 1 when the voltage applied tothe source of light was 11 V (the relative value is hereinafter referredto as “relative value of the electric current ratio”).

INDUSTRIAL APPLICABILITY

The photoelectric conversion element of the present invention exhibitsexcellent photoelectric conversion properties, especially highefficiency in converting solar energy to electric energy (i.e., highenergy conversion efficiency), and a dye-sensitized solar battery can beeasily produced therefrom. Therefore, the photoelectric conversionelement of the present invention can be advantageously used for adye-sensitized solar battery and the like.

1. A photoelectric conversion element comprising a composite dye and ann-type semiconductor, said composite dye comprising a plurality ofcomponent dyes which have different excitation levels and which arechemically bonded to each other to form a straight chain or branchedstructure for transferring an excited electron therethrough, whereinsaid straight chain or branched structure is, at one end thereof,secured to said n-type semiconductor and has, at least at one other endthereof, a free end, wherein, in said straight chain or branchedstructure, said plurality of component dyes are arranged in an ordersuch that the excitation levels of said plurality of component dyes aredecreased as viewed from said one end of said structure toward said atleast one other end of said structure, wherein each component dye ofsaid composite dye comprises a metal atom having a ligand coordinatedthereto, so that said composite dye is comprised of a multinuclearcomplex comprising a plurality of metal atoms and a plurality of ligandsincluding at least one bridging ligand, wherein the or each bridgingligand is positioned between mutually adjacent metal atoms in saidmultinuclear complex to thereby bridge the mutually adjacent metalatoms, wherein the or each bridging ligand in said multinuclear complexcomprises a heterocyclic segment having a conjugated double bond and,bonded to said heterocyclic segment, a non-heterocyclic segment, tothereby form an asymmetric structure, wherein the heterocyclic segmentis positioned in the or each bridging ligand on a side thereof remotefrom said n-type semiconductor as compared to the non-heterocyclicsegment, wherein a heteroatom in the heterocyclic segment is positionedon a side thereof remote from said n-type semiconductor.
 2. Adye-sensitized solar battery comprising: an electrode comprised of thephotoelectric conversion element of claim 1, a counter electrode, and anelectrolyte interposed between said photoelectric conversion element andsaid counter electrode, wherein said dye-sensitized solar batterybecomes operable when said electrode comprised of said photoelectricconversion element and said counter electrode are connected to eachother through an electroconductive material which is positioned outsideof said electrolyte.
 3. The dye-sensitized solar battery according toclaim 2, wherein the potential of said counter electrode is −0.2 V ormore relative to the redox potential of silver/silver ion.
 4. Aphotoelectric conversion element comprising a composite dye and ann-type semiconductor, said composite dye comprising a plurality ofcomponent dyes which have different excitation levels and which arechemically bonded to each other to form a straight chain or branchedstructure for transferring an excited electron therethrough, whereinsaid straight chain or branched structure is, at one end thereof,secured to said n-type semiconductor and has, at least at one other endthereof, a free end, wherein, in said straight chain or branchedstructure, said plurality of component dyes are arranged in an ordersuch that the excitation levels of said plurality of component dyes aredecreased as viewed from said one end of said structure toward said atleast one other end of said structure, wherein each component dye ofsaid composite dye comprises a metal atom having a ligand coordinatedthereto, so that said composite dye is comprised of a multinuclearcomplex comprising a plurality of metal atoms and a plurality of ligandsincluding at least one bridging ligand, wherein the or each bridgingligand is positioned between mutually adjacent metal atoms in saidmultinuclear complex to thereby bridge the mutually adjacent metalatoms, wherein the or each bridging ligand in said multinuclear complexcomprises a heterocyclic segment having a conjugated double bond and,bonded to said heterocyclic segment, a non-heterocyclic segment, tothereby form an asymmetric structure, wherein the heterocyclic segmentis positioned in the or each bridging ligand on a side thereof remotefrom said n-type semiconductor as compared to the non-heterocyclicsegment, wherein a heteroatom in the heterocyclic segment is positionedon a side thereof remote from said n-type semiconductor, and wherein,when said multinuclear complex is irradiated with light rays, anelectron-deficient state shifts from an electron orbital ascribed to ametal atom having a higher energy level to an electron orbital ascribedto another metal atom having a lower energy level.
 5. A photoelectricconversion element comprising a composite dye and an n-typesemiconductor, said composite dye comprising a plurality of componentdyes which have different excitation levels and which are chemicallybonded to each other to form a straight chain or branched structure fortransferring an excited electron therethrough, wherein said straightchain or branched structure is, at one end thereof, secured to saidn-type semiconductor and has, at least at one other end thereof, a freeend, wherein, in said straight chain or branched structure, saidplurality of component dyes are arranged in an order such that theexcitation levels of said plurality of component dyes are decreased asviewed from said one end of said structure toward said at least oneother end of said structure, wherein each component dye of saidcomposite dye comprises a metal atom having a ligand coordinatedthereto, so that said composite dye is comprised of a multinuclearcomplex comprising a plurality of metal atoms and a plurality of ligandsincluding at least one bridging ligand, wherein the or each bridgingligand is positioned between mutually adjacent metal atoms in saidmultinuclear complex to thereby bridge the mutually adjacent metalatoms, wherein the or each bridging ligand in said multinuclear complexcomprises a heterocyclic segment having a conjugated double bond and,bonded to said heterocyclic segment, a non-heterocyclic segment, tothereby form an asymmetric structure, wherein the heterocyclic segmentis positioned in the or each bridging ligand on a side thereof remotefrom said n-type semiconductor as compared to the non-heterocyclicsegment, wherein a heteroatom in the heterocyclic segment is positionedon a side thereof remote from said n-type semiconductor, wherein, whensaid multinuclear complex is irradiated with light rays, anelectron-deficient state shifts from an electron orbital ascribed to ametal atom having a higher energy level to an electron orbital ascribedto another metal atom having a lower energy level, wherein saidmultinuclear complex has a structure represented by the followingformula (1):(L₀)_(λ1)(X₀)_(λ2)M₀[(BL)_(m){M(L)_(n1)(X)_(n2)}_(pm)]_(q)  (1) whereineach of L₀ and L independently represents a ligand having a heterocyclicsegment which can be coordinated to a transition metal atom; each of X₀and X independently represents a ligand which does not have aheterocyclic segment; each of M₀ and M independently represents atransition metal atom; BL represents a bridging ligand having aplurality of portions, each of which can be coordinated to a transitionmetal atom; λ1 is an integer of from 1 to 7 and λ2 is an integer of from0 to 6, with the proviso that the sum of λ1 and λ2 is not more than 7; mis an integer of from 1 to 7, with the proviso that the sum of λ1, λ2and m is not more than 8; n1 is an integer of from 0 to 6 and n2 is aninteger of from 1 to 7, with the proviso that the sum of n1 and n2 isnot more than 7; and each of p and q independently represents an integerof 1 or more, wherein when each of λ1, λ2, m, n1, n2, pm and q is aninteger of 2 or more, the plurality of L₀ is the same or different, theplurality of X₀ is the same or different, the plurality of BL is thesame or different, the plurality of M is the same or different, theplurality of L is the same or different, and the plurality of X is thesame or different, wherein the heterocyclic segment of the or eachbridging ligand BL is a multidentate segment having a dentate number of2 or more, wherein the or each ligand L₀ is a bi- to quadridentateligand.